You're restricting until the population is vaccinated because:
- Increased socialisation = increased transmission;
- The vaccine is not perfect.
- We have yet to have it confirmed that being vaccinated stops, entirely, transmission.
So, therefore, until everybody has been vaccinated, we still offer up a risk to elderly, infirm populations, and offer a risk of increased illness, and hospitalisation, for younger groups. That, therefore, still offers a risk to the NHS being overwhelmed and unable to function 'normally'.
Once everybody has been vaccinated, then we reach a position of the fabled herd immunity, given that some people won't take it, anyway. At that stage, there is a risk, but a realistic risk compared to everything else in life. Before that stage, you're mixing a number of people who haven't been vaccinated, with those with a certain %age of protection.
As for third wave I'd trust the chief medical officer, with years of training, when he considers the future, and what is to come.