Is Buddhism in a worldwide decline?

I don't have statistics for you but I do want to point a few things out to give you hope against the dire (skillful means) predictions of a Dharma Ending Age:

• Statistics lie. There must be a huge amount of "unnanounced Buddhists" who practice meditation with the quest for Awakening (freedom from suffering). This to me qualifies as Buddhist, especially because some of these "untracked" people spend far more hours in the day practicing the essence of Buddhism than those affiliated to a softcore branch of Buddhism.. I myself am one of those people who announces themselves as multiple religions/none.
• Buddhism is more of a spiritual lifestyle than a '-ism'. Therefore anyone of any religion can practice it in the cover of their communal Abrahamic (or otherwise) religion.
• Buddhism can never die. The quest for freedom and happiness will never end and due to the information age, the Buddhist teachings/suttas are forever preserved even if a major portion of humanity is decimated.

So long as you yourself are interested in Buddhism for its life-benefitting knowledge, you can be sure many others are, too.

In the modern age of world-wide education & science, all religions that have superstitious beliefs will inevitably fall into decline (statistically). Asian cultural 'Buddhism' is no different to Western Christianity or Judaism. Many Asian cultural Buddhists will stop believing in unknowable superstitious ideas (such as 'reincarnation-rebirth' & external 'heavens & hells') & thus reject (what they believe) Buddhism (to be).

The Buddhist scriptures (such as MN 95) report the Buddha criticised religions that adhere to blind faith yet, since that time, Buddhism has become the same. For example, today, there are famous teachers that will insist you cannot be a Buddhist if you do not believe in post-mortem-rebirth.

"Master Gotama, with regard to the ancient hymns of the brahmans — passed down through oral transmission & included in their canon — the brahmans have come to the definite conclusion that "...this is true..." What does Master Gotama have to say to this?"

"Tell me, Bharadvaja, is there among the brahmans even one brahman who says, 'This I know; this I see...this is true..'"

"No, Master Gotama."

the Dhamma... is deep, hard to see, hard to realize, tranquil, refined, beyond the scope of conjecture, subtle, to-be-experienced by the wise..

I speculate Buddhism only historically grew as a cultural religion due to a change of emphasis towards 'rebirth-reincarnation' (which was also the primary reason why Buddhism became extinct in India due to it becoming undifferentiatable from Hinduism). Therefore, it is inevitable cultural Buddhism declines for the same reason that it grew; in that it was never ever True Buddhism.

In conclusion, (in ordinary circumstances) the True Dhamma will not decline since those that comprehend Ultimate Knowable Truth do not change their convictions.

Main aspect leading to Buddhist decline or growth as per The Dharma-ending Age :

1. the admission of women into the monastic community;
2. the lack of respect toward various elements of the Buddhist tradition;
3. the lack of diligence in meditation practice;
4. carelessness in the transmission of the teachings;
5. the emergence of divisions within the Sangha;
6. the emergence of false or “counterfeit” Dharma; and
7. excessive association with secular society.

Later texts also mention external causes of the religion’s decline of which there are two:

8. the invasion of India by foreign, non-Buddhist powers; and
9. excessive state control.

Out of these we already see many elements today. So naturally Buddhism is in the decline. The way to get over this decline can be as follows:

• Every Buddhist should operative the 4 frame of mindfulness diligently to attack 3
• Should teach this diligently and sharing Dhamma realized empirically to attack 2
• Should try to source as much from the Suttas strengthened though empirical understanding to attack 4 and 6
• Be mindful of the Vinaya and how the Sangha is organised without trying to bring variations 5, 7 and 2
• Look at the Dhamma as it is without a mixing personal or cultural beliefs to attack 2 and 7

In addition to the other answers, I believe there is a global decline in many major religions as you can see in this Google trends graph.

I would argue that especially in the western countries choice and interest of religion is subject to change, e.g., due to trends and/or major events.

According to the commentaries, Buddha Sasana will last 5000 years. That means about another 2400+ years. In the 1st millennium, there were Arahaths with special Jhanic powers. In the 2nd millennium, there were Arahaths with no specials powers. In the 3rd millennium, i.e. at present, it's said that there are only Anagami Sangha at most. In the 4th millennium, there will be Sakadagami Sangha at most. In the 5th, there will be Sothapanna Sangha at most. At the end, monks won't even be wearing robes. They will wear ordinary clothes and will just use a yellow string to indicate that they are monks.