Do any historians consider superior German airpower as the most important factor in the Battle of France?

I usually consider that the main factor in German victories of 1940 and 1941 is the hability to concentratre fire power. This firepower came from artillery as well as air forces. The armoured forces played a side role compared to this ability, except for some operations in Russia where they were able to isolate Soviets units so fast that those units broke down.

Speaking about the specific Battle of France, German air power was of significant help because the French army put itself at risk with its own plan: the bulk of the forces advanced to the North while the Germans planned to breakthrough at Sedan, in the South of the front line.

In this situation, the capability of the Luftwaffe to interrupt the mobility of French reserve forces and the reaction of the "heavy forces" engaged in Belgium played a decisive role in reducing the mass and the firepower of French counter-offensives. As a result, the French were not able to cut the German breakthrough, even if in theory they had powerful/numerous enough field guns (even if only 75 mm) and tanks to succeed in such a counter-attack.

However, as the war went on, Germany lost its upper hand in firepower:

• Air forces were countered
• Artillery could not suffer the losses it has been inflicted, such as during the Kalinin offensive, because it was not predominant in production

Good question. Modern doctrine would say AirPower is paramount for massed armor offensive as the Germans used against France in 1940. Today we know Armor mobilized is armor exposed and susceptible to attack by air. The reason why German air wasn’t given the primary credit at the time was because every aspect of the tactics the Germans were using was so novel and impressive, the allies didn’t know the counter.

Nobody had used massed armor like that before, nobody had used coordinated air in support of ground units with such effect before. And of coarse nobody knew that massed armor could be smashed by AirPower as the allies would demonstrate to Rommel in North Africa. In France the British and French air didn’t have the training or understanding to attempt to counter the Germans which diminishes the credit and importance of the luftwaffe role.

The Germans so outclassed the allies in tactics, training, and general showing it was easy to credit the entire performance and not isolate to specific components of the offensive such as air.

So dominant was the German performance in the battle of France it was easy to overlook the role of the luftwaffe. General Von Rundstedt who was in the battle of France in command of battle group A would demonstrate this several years later in preparation for the defense of France. Von Rundstedt would argue to reserve the best German units away from the beaches for a counter attack to an allies invasion of d day. Thinking he could recreate the successes of blitzkrieg after the allies landed even though the luftwaffe no longer controlled the air. Rommel with his experience in North Africa would argue without air superiority units not at the beaches would be destroyed before they could be brought into effect. Von Rundstedt a German field marshal four years after the battle of France still failed to grasp the importance of air superiority.

In the Battle of France, the Allies and Germans had roughly equal number of divisions and troops, and the Allies actually had more artillery and tanks. But the one area where the Germans had a clear superiority was air power, with roughly a two to one edge.

The one thing that should be said first is that the Germans had air superiority, but just as with tanks, men and artillery, their air power was not superior in numbers overall. It was just better focused to achieve local superiority where needed.

The allies could field roughly 1.500 fighters and bombers combat ready along the border to Germany, while the axis had around 2.500 ready to attack. But in absolute numbers, the allies had about 4.000 and the axis 3.000, that's almost the opposite in terms of air power. As always, the axis was outnumbered in absolute numbers. So it came down to mostly what the whole campaign was planned around and came down to: concentration of force on a specific goal, tactically and strategically. When in a war for more than 6 months, there is really no valid excuse to have more than half your air force stand down and parked behind your lines. But the allies did. I haven't read enough literature on this to say with certainty whether this was due to personal incompetence of the military leadership or just obsolete strategies, but it fits in really well with that mix that basically describes the whole campaign on the french side: "Unprepared for modern warfare, fighting the war of 1918 when it's 1940", still believing that stretching your forces out is good and your "line" being flanked is the worst thing that can happen.

But could these successes (and the failure of Allied countermeasures) be at least partly attributable to German air superiority?

Yes. The air force did play a big role in the blitzkrieg/schwerpunkt/combined arms tactics of the Germans. Self-propelled artillery came later on, so much of it was still towed and took long to set up. Air power was a good and sometimes the only way to crack tank formations and bunkers.

Looking for a potential turning point of the campaign due to air power of either side, there really was only one big air battle: Sedan 13/14th May. The Germans were about to cross the river with a pontoon bridge at Gaulier and the allies knew that that river was the best defense against the German tanks they had. Once the tanks were over the river, they'd not be able to contain them. So the french generals (correctly) decided to focus all their air power on that bridgehead and more importantly the bridge itself, so no more troops would cross. They called it "la fournaise de Sedan" (I guess that translates to "the furnace of Sedan") meaning a kind of hellish, fiery doom kind of place. However, unlike the Germans, they simply weren't able to focus their power that effectively. Throwing "everything" at it resulted in about 400 planes attacking, less than half of them bombers. The french fighters flew 250 sorties to protect the bombers. That's nothing compared to the 814 sorties German fighters flew to defend those bridges and the 300 AA guns they had amassed at that place. That air battle was a total disaster for the french with not a single bomb hitting the bridges and more than 150 planes lost (shot down, crashed or returned but beyond repair).

So yes, even if you don't consider the offensive capabilities, the campaign could easily have been lost of not for the defensive capabilities of the air force and AA regiments. Ensuring that victory is possible and the troops can be effectively deployed (across a natural boundary in this case) and used is a very strong contribution to any war effort.

I've played a number of board games regarding France 1940 where "tactics" largely cancel out, but the one thing that stands out is the superior German airpower.

I think that's basically because "reality" would be boring to play. A depiction of reality would be the allies player have double the number of pieces of every kind, but the axis player taking three turns in a row for every allied turn, thereby being able to focus his fewer pieces into a situation where he has superiority at the one situation he picks every round. It would play a little bit like tower defense on the french side. You set up your pieces and your enemy takes turns while you can do nothing but watch and pray that it holds.

Has any historian(s) made the point that superior airpower, was the decisive (not merely contributing) factor and sina qua non to the German victory in France in 1940?

I don't know of any and I don't think it would be fair to do that with any factor. The decisive element was not a single one, but rather the successful combination of all of them. As in any good team if they work together, they get way stronger than their parts and can overcome opponents that when laid out on a datasheet seem superior in every single category.