That's just not going to happen as we will eventually get to the point that games are just too much for the regular PS4 and XB1 to handle. If I was a betting man I would say we're not too far from that point either considering how fast technology evolves.
If I was a betting man (and truth to be told, by all accounts I am), my money would be on Microsoft eventually introducing OS iterations as the significant factor, and discontinuing older platforms by ceasing to provide firmware updates when the hardware becomes too obsolete. Of note is the fact that developers could easily choose what would be the "minimum requirement" for OS version, and set their price point accordingly - this way, we could have the most barebones indie games of 2040 still coming out on the good ol' Xbox One, while the newer AAA games with 24k resolution, SHDR, simulated haptic feedback and holographic 4D support could release for the Xbox Borgcube 4000 or whatever the remote descendants of the current platform will be named.
"Have I already heard something like that?",
yes, my good sir, indeed you have - that is essentially how smartphone "generations" work already. Sure, consoles may be more specialized devices, and the high price point for most of its software limits the flexibility Microsoft'd have with an "Xbox OS" series of products, but they follow the same "plug-and-play and buy another x years from now" marketing model as the iPhone. The concept of "generations" as we know it would go the way of the dodo, MS could potentially sell more Xbox-es, and at the same time development costs would go down; players would lose little, as the inter-generational technological gap has gone down significantly already (and in fact, there are many engines which survived the transition between the 7th and 8th generation without any significant alteration, Crytek's CryENGINE being one of the most notable examples).
In all of this, Forza can evolve over time instead of completely abandoning an old-and-proven infrastructure for a brand-new one when new hardware becomes available every eight-or-so years, like when the Xbox One superseded the 360. I don't think the quality of the assets will be a problem anytime soon, either - poly count for 8th-gen models is, in all likelihood, significantly higher than the 170-200k polygons for a LOD0 model we had in FM4; and changing the quality of textures is ultimately something that can be done without much of a waste of time.
This is the one that came first to mind considering some of the cars that looked really poor in FM5, were FM4 models. So obviously, they have been able to carry over assets easily. It should be a requirement though, that they go through a re-touch to bring their quality up to the newer cars. The Saleen S7 has looked really shady for years now & I'll bet it still looks off in FM7.
I'm not sure those poor quality models were straight ports from FM4 - undoubtedly, even the shadiest car got a bump in its poly count in the transition from the 360 to the One. That probably means that they have high-poly "resource" models stacked somewhere, and they just produced new "working" models with a less aggressive use of whatever's the equivalent of the Decimate tool in their no doubt specialized software.
In the case of the 22B, they mentioned how they got hold of an Impreza and re-scanned it from scratch - although of course until we'll see it in FM7 as a completely new car, there's no way of saying if it's just marketing talk (which is a possibility I'm not ready to discard just yet).