So we agree that UWP has good features. That's not under dispute.
The problem is that it creates an unfair competitive environment. Steam can't really use these features because it has to instruct each individual user on how to turn 'sideloading' on.
Indeed, there is a reason antitrust laws exist. Allowing companies to leverage a monopolistic position in one area (o/s) to create a monopolistic position in another which the company had no previous advantage in (stores) is not good for competition, diversity of choice, the consumer or society.
The problem is that it creates an unfair competitive environment. Steam can't really use these features because it has to instruct each individual user on how to turn 'sideloading' on.