Lock-in != Monopoly.

The fact that you can’t transfer your purchases […] to other platforms

This is ridiculously unrealistic in a capitalist society.

It costs the platform money whenever a user downloads a game, and a user who didn’t buy from their store isn’t a user that they make money from. No other platform would voluntarily accept a recurring cost like that.

Also, it’s not like they stop publishers from doing that themselves. Ubisoft and EA use the cd-key generated by steam to associate the game with your U-Play and Origin accounts.