Two versions of a game will look/run differently in the same ecosystem. Some games will not release on the weaker platform if the game is really demanding. And many people will see this and decide their purchase is not good enough and buy a second console, which is basically near identical to the one they have and pay premium for that.
Consoles are not PCs, you buy it expecting it to last as many years as possible. You don't want to spend that much money every couple of years, mobiles are bad enough for that, I don't want that business model for consoles. If you don't mind upgrading every now and then, you get a PC.
No, demanding games won't skip the base PS5 which is mandated, it's one of the few things that still set consoles (generations) apart from PC, for better or worse.
Mid gen upgrades existing literally will not affect your experience compared to if they wouldn't exist, unless Sony allow 3rd parties to skip the standard model which they will not (3rd parties will not want to skip it either way because it's crazy popular and will easily outsell the Pro). Until they do, you're not affected in a way that is dissimilar to pricedrops and deals for games and consoles. Neither PS4 Pro nor Xbox One X had games that couldn't run on the original console, and I don't expect this to change on the PS5 side (it may however change on the Xbox side because the Series S is underpowered from day 1). PS5 will power through the generation no problem, just with lower resolutions and settings. Hell, a lot people here seem to think even the Series S will be fine.