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.
I had the same 'fears' when ps4 pro and One X came out. But it turned out not to really make development harder, the pro consoles were just used to render at higher resolution / fps with a couple effects cranked up. With the exception of CP2077, everything worked perfectly fine on the base PS4. But yes, some multi-plats like Ark and Rust definitely ran a lot better on the pro.
Since developers are already relying on VRR to support their array of modes they like to provide this gen (quality, performance, RT) I'm fine with a ps5 pro coming. It will give devs more options to pursue game play advances if they can spread the modes over the consoles. Just like Series S and Series X, yet with PS5 as the base console and the pro for the 4K and 60 fps bells and whistles.
All games will still release on PS5, just like the all still released on PS4 after the pro came along. You can safely use the ps5 until the end of the generation. Only now Horizon FW is skipping PS4 (and pro) with its first DLC. There are zero games that only run on ps4 pro. You don't have to upgrade. However game development has not reached the end yet, and every generation we've seen performance get weaker towards the end of the generation simply due to developers pushing the hardware harder and harder to keep advancing games. With pro consoles you have an option to play them at higher settings instead of waiting for a next gen remaster...
And now gens are fully BC again, you get all the benefits of the 'pro' enhancements automatically. If you skip on the pro consoles, you still get all the benefits of it in the next console generation.
If you don't want to spend that much money, play last gens games. They perform wonderfully in BC mode, are fully complete, bug free, and cheap! Console generations keep getting longer, original PS1 lasted 5 years, original XBox only 4 years. A refresh after 4 years for an 7 or 8 year console generation, seems a win win for everyone. You still get 10 years of support of the original model, but don't have to resort to PC to still get decent performance on PC ports towards the end of the generation.