A 2005 vintage PC? A 32bit Pentium 4 Netburst with at most 2GB of RAM? No SSD, no GPGPU, no unified shaders?

How many people are willing to spend 60USD+ for a AAA game to play on a computer worth 60c?

How many people will then purchase cosmetic microtransactions when the computer renders the player character as an orange cube?

Is this worth $100,000s worth of development cost to architect the software (which may involve a third party engine) that can scale across everything without compromising the ability to play the game? It isn't simply a matter of making textures single colour and replacing all models with cubes. Consider in-game physics. Does massively simplifying the model compromise the ability to complete the game?

Can you even have multiplayer games with players running two different physics models?

Interesting idea. Would be fun to experience some games in a "classic era" look, by easily accessing that option in the menu. Also, I often wish that some titles would run on my system, I've never owned current hardware.

Apart from some of the good reasons already mentioned, I'm guessing that developers are worrying that their vision of the game in terms of atmosphere/immersion would suffer when drastically changing the presentation.

long and short of it, there's little point.

Double potato à la mode recipe: be a couch potato while playing a Switch potato mode game.