Until someone builds a Steam that does allow it, and users start migrating over, and steam has to either allow it or keep losing market share.

Companies don't do things out of the goodness of their hearts, they do it because better, faster, more consumer friendly competitors appear.

That might not be up for Steam to decide, first-sale doctrine already exist as law in many countries, what's missing is the enforcement of it for digital goods:

https://www.rockpapershotgun.com/steam-should-let-users-rese...