GP’s point was that you don’t need to close your Steam account because it doesn’t cost anything: you only pay for the individual games, not Steam itself.
EDIT: I believe GP is incorrect about the nature of subscription fees for PS+ and Xbox Live, though. As far as I know, standalone purchases of games from those services do not ever require a paid subscription - you need to retain your account and connectivity for the license checks, but that's free and does not require a paid subscription on either service, so pretty much the same as Steam. But they are correct that those platforms don't provide other store options. EDIT 2: Ah, I misread GP, they said "online content", and maintaining subscriptions on those services is required for that.
It's not any form of lock-in or anti-competitiveness, and it's not an aspect that's specific to Steam. You actually need to substantiate that instead of just claiming it. Almost all the online digital platforms do this, even non-gaming ones, and it's weird that it's only being argued here because it's about Steam.
This is a digital media rights issue, not a Steam issue.
AFAIK your games are locked to your account, and you'd need to log in at least once a month or something to keep access even in offline mode.
If you close your steam account you lose everything, so I don't see how that's a weak form of lock-in?