The problem being (as I see it) is that if these case numbers keep rising as they are (200,000 a day by Christmas) then its not just this new variant they are worried about but the masses of people self isolating for 10 days each time and the impact on the actual services. Hospital beds may not be at crisis point, but the staff to actually care for them might be.
What about the rozzers, fire service, supermarket staff, etc, etc if half of them can't actually work?
If they do nothing and let it spread then workplaces will be massively understaffed throughout January.
The booster gives extra protection hence the big push on it.

Thats my view anyway.