The XBox has been allowing sideloading for a while now (XBox One and now Series X) and it hasn't caused any problems. XBox Live Indie Games for 360 was also not an issue. Sony had sideloading for homebrew on PlayStation at various points in time too.

I think making it a required policy (with allowed constraints) would not be harmful to consoles at all.

Requiring third party stores is more difficult, but honestly I don't think most people would need them if you can just sideload.