I think you guys sum it up quite nicely between you. While decent copies have moved out of reach of many, some will react by getting the best low-grade copy they can afford, and some will consider the cost and opt for a nicer shape copy of a different SA key that they think has legs. The fun here is in the speculating about it. Some will turn to ASM 1, I'd think. Some, like Marmat in his post, will target an alternate subgroup of SA keys. Others will move to what they think is the next frontier for big % gains, be it a different era, the next tier of SA keys, etc.
That's among the many reasons I love this hobby; it's got such a diversity of niches/micro-markets; the options are endless. (Not that SA keys are a micromarket, but the alternatives for those who consider themselves priced out of any they would want include many options.)
i also like zhamlau's analysis in that wanting a book and affording are not the same thing. People who want the book but are priced out also keep the floor high if something falls close to their price range. A character has to become pretty irrelevant to really crash in price, and that can and does happen. But Spidey in the foreseeable future? Seems more likely a slowing than a reversal-type crash. (To get squarely back to the original question.)
Also consider: every priced out collector is one good find away. People find, buy and sell underpriced items all the time, funding their collecting goals in the process, no?