Just my theories...
High prices for starter/project cars, or even middle-of-the-road "drivers", is bad for the hobby because it does indeed limit the number of folks who can afford to "buy-in".
High prices for finished cars, done to a high standard, is not bad at all for the hobby, if you look at it with a wide economic lens. First off, no young person with limited funds is expecting (or should expect) to buy a finished/restored Mach 1 or high-content restomod as their first foray into the classic car realm. Secondly, low prices for FINISHED cars means that the hobby is, in reality, less-affordable than it is when the prices are higher for finished cars. Why?? Who the heck can "afford" to be horrifically up-side-DOWN in their Mustang? It is shortsighted to look at prices of finished cars, and consider it a negative. It's the opposite. That means that a person of modest means can indeed afford to get a fixer-upper, do the car the way they want it, to a high level, and then... If the necessity arises and they have to sell it, they don't wind up losing their shirt (and likely their wife) after the dust settles and they realize that they just set $30K on fire by virtue of participating in a hobby.
High prices on finished/good/desirable cars fuels more restorations, because the restorations are more-economically-viable, long-term, regardless of your bank balance.
If restomods seem to be lingering longer for sale, that might be an indicator of something, but too early to tell. It's always been a reality in the street-rod realm, that it's tough to sell one for anything close to what you've got in it, because all the street rodders want a car done "their way", and it never quite matches what "your way" was. So the purchase price is handicapped by the buyers' anticipated expenses of making the car reflect their own desires.
Restomodding has been good for the hobby because its fueled the saving and restoring of a ton of 6-bangers and 2-barrel cars that prior to the movement, tended to either sit idle, or just weren't "invested in" to near the extent they are now. So it's all been good.. But it'll all turn to bad if all of a sudden the well-heeled guys aren't willing to write big checks for them, and they all wind up being significant losses to their owners (or the owners descendants) when they eventually change hands.
All that said, I think the healthiest hobby will be one where projects are valued more realistically, properly taking into consideration how much has to be invested to get anywhere worthwhile with it. And the finished products are valued commensurate to the costs to get them to that point, give or take. I also think things are healthiest when you've got a balance of original restorations and resto-mods. Without a strong population of correct/original cars, the Mustangs' position as a "historic artifact" gets significantly eroded, and that really hurts values, for ALL Mustangs. If your radical restomod lives in a world where 100% concours-correct examples aren't plentiful or popular, then the "aura" and "mystique" of your restomod is diminished. A car is coolest when it's based upon a car that's cool to begin with. So the original cars need to remain cool and valuable in order to maintain the excitement for the modded ones.
I blab a lot folks, I know
But these are the things that keep me up at night!