> that older iOS and macOS versions end up unable to install any software that hasn't had an older version archived somewhere

iOS, sure. After a certain point, you need to be fine with simple functionality (but, I’ll add, more than adequate for most users, which means someone else could find use and value for what you treat as junk).

But macOS? What software? Everything I’m thinking of is graphics adjacent and significantly benefits from faster hardware. For almost everything else, a browser suffices.