I say the day Mozilla bends the knee to advertisers and do what Chrome did by phasing out ad-blockers with Firefox. That'll be the day when I look around my apartment and realize "man, I need to start getting into some hobbies now."
Depends on what in using it for. There is no way to fully avoid the Internet anymore. You can't interact with businesses or government without it. So, for that, we're stuck with it.
I work in IT so I'll be stuck with it there. I already have no expectation of privacy at work, though.
For entertainment and engaging with communities like Lemmy, I think losing anonymity through mandatory online id would be the nail in the coffin.
I already moderate myself on social media, but I have an outlet in these communities. I would simply stop using them if I lost that.
I would guess underground networks would rise up to replace them, but I would not want to put myself at risk. I don't trust VPNs enough for that.