You seem to have a fundamental misunderstanding, here. You can sell Steam keys on other platforms. Plenty of developers sell Steam keys on Itch, for example.

You seem to have a fundamental misunderstanding, here. You can sell Steam keys on other platforms. Plenty of developers sell Steam keys on Itch, for example. You get to keep 100% of the revenue from an off-Steam Steam key sale. 100%! The only limitation is that they don't want you selling the keys for peanuts on other platforms. If your other platform is handling distribution, you can go nuts. Just don't charge 50% less on other platforms if you are depending on Steam for distribution.