I don't think herd immunity is the sole bar for lifting restrictions. As you approach herd immunity, spread and new cases will slow as the bug finds it harder to find new hosts. No US politician has been able to implement a "zero covid" bar, they'd get lynched (New Zealand managed, but they've got very big moats and a socialist society). As long as the health-care system isn't overwhelmed, and it's not trending there, it's going to be hard to maintain restrictions just because.
Also... the at-risk population will have first access to vaccines, and the most motivation to actually get one. That will reduce both spread and case severity disproportionately to their % of the general population.