The Issue is the Authority of Scripture and How We Should Read It

Not all YEC think alike, but some of the most compelling arguments I've heard regarding this have to do with why it's desirable to someone to make the account metaphorical. If one is just trying to make the Biblical account harmonize with the latest naturalistic theories concerning the age of the universe (or any other contemporary hot-button issue), then your interpretation (of any portion of scripture) is likely to suffer from a failure to listen (to God). It is seen as placing restrictions on what the passage must mean before you listen to what it's saying. It seems kind of backwards to have a naturalist/materialist tell you how you should interpret the Bible based on conclusions that depend on his invalid assumptions (the non-existence/non-importance of the supernatural).

Though you have dismissed it, the argument goes that if the creation account isn't meant to be taken literally, then there is not a clear place to draw the line as to which parts of the Bible are metaphorical and which are literal. The Genesis account has the style of a historical narrative. Adopting such a exegetical strategy is considered to erode the authority of the Bible because it weakens any defined standard for how to honestly and accurately interpret the text. I don't know about the gospel and salvation crumbling directly from a metaphorical creation account, but the usefulness of the text to the reader suffers if it cannot be assumed that the parts that sound like (literal) historical narrative need not be understood literally.

This is only an observation, not an argument, but in the case of Genesis 1-2, what most metaphorical interpretations I've heard don't do very well is tell you what it actually means. They usually just find a way to work in long spans of time into the account. They don't offer much in the way of explanation for the amount and kind of detail that is recorded. I've seen a lot of exegetical acrobatics employed to explain why it can't be literal, but I haven't heard more than a superficial explanation as to what its metaphorical meaning was to the original audience or to modern readers. It doesn't seem like a very satisfying alternative to read the Genesis account that way. It deprives all but the abstract thinker the ability to understand what's written, and an implicit assumption by a number of literalists is that the God's wishes (and, consequently, the Bible) can be understood by everyone.

An extremely simple argument for this sense is to consider the following two questions:

Q1. Does the epistle of Romans assume and rely on an essentially literal* interpretation of the fall of man (cf. Genesis 1-3)?

Q2. How foundational is the epistle of Romans to a Christian understanding of the gospel?

It may be possible to argue these points to a certain degree from a non-evangelical perspective, but for the vast majority of YEC's the answers are evidently (and respectively):

A1. Unambiguously yes.

A2. It is critically important - our understanding of the gospel would be seriously undermined without it.

Conclusion: An essentially literal* interpreation of the fall of man (as seen in Genesis 1-3) is critically important to our understanding of the Gospel.

edit: Another issue that occurred to me regarding this: if you do not link (the initiaing of) physical death with the (inital) instance of sin, it significantly undermines what sense came be made of the Atonement in particular** - how can Christ's physical death have any impact on sin and it's consequences if the consequences of sin do not produce physical death (but are just the result of natural processes)?

*by this I mean that the 'essentials' of the story (no death before the fall, death comes as a consequence of the sin of the progenitors of the entire human race) are literal, overly literalistic obsession on minor details is not necessary.

**also (OT) atonement more generally - why should we have "without the shedding of blood there is no remission of sins"?