You can add non-steam games to steam if thats what you mean. Theres a launcher called hydra which does, things. Playnite is a launcher I use that I plug all my other accounts in to and launch all games thru that. There are also ways to unlock dlc and stuff but that will require a little bit of research. Havent sailed the seas for games in a while.

TLDR; Yes you can add otherwise aquired games to steam. For the most part

You mean this?

Don’t you think app developers should have the freedom of choice for app stores? Android has multiple places to get apps, lots of FOSS apps aren’t on play store, and likely aren’t on iOS at all. Having the option for 3rd party store gives not just users, but developers too, an option and access to more apps/users that wouldn’t otherwise be available. You want safe secure apps, that’s all good.
You mentioned that an app might move away from the app store to a 3rd party store, if you already use and trust that developer’s product how will having to get it from a different location change that?
Now, not to sound too harsh, but why your convenience of not having to go get another store more important than a devs ability to host their product in a different location?