That's a weak form of lock-in though, because you can switch to another platform going forward, and access your previous Steam purchases for free. No ads, no subscription fee. Stay signed out of Steam Chat and the social stuff, and you've basically just got a heavy application launcher.
Compared to PS+ and Xbox Live, which charge subscription fees to continue accessing online content, it's a pretty sweet deal for the consumer.
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.
Compared to PS+ and Xbox Live, which charge subscription fees to continue accessing online content, it's a pretty sweet deal for the consumer.