The main difference is that if steam went offline rn, if you download it off itch, the game is yours to keep forever. Steam is great don't get me wrong, but if you can download like the official .exe file I would do it that way and move it to steam. Remember steam doesn't give you the official .exe of the game so if steam went offline EVERY SINGLE game you have ever paid off steam is lost forever.
I wish I could say you are wrong. But:
1, Steam *has* a "kill-steam" switch, and it's tested, and works. Valve is capable of removing all dependencies on the steam webservice itself, so if it does go out of business or otherwise die, your launcher will still work. This is good.
2, But this only applies if you are able to run the *current* version. If, for example, you had an older mac, and you explicitly purchased games that would run on your older mac, then you might find that steam forced an update to a newer version *of the steam client* that won't run on your system, and suddenly you are unable to launch the games you purchased on the older system.
3, And that has already happened to windows -- you have to run the yucky win11 to run steam and launch programs, even if you purchased them for your older XP system. Ugh.
Yes, I go for Humble, or GoG, or Itch, or ... anything else if there is a choice.