The theory is that once the lockdown is finished, the virus spreading will go exponential again. In which case, you need another lockdown. And so on.
And no country has the economical capabilities to stay in lockdown until end of 2021, when we can assume mass vaccination will be possible (maybe a bit earlier like summer 2021).
I am not saying that I think this is the case, but this seems to be the logic behind it. And well, I am a bit afraid that they are right, otherwise UK's plan (and US, Netherlands, Sweden) is nuts. I am afraid that they have checked all the possibilities and have come to that conclusion.
This is largely it. Long term, there are only two controllable ways that this pandemic gets under control: vaccine or herd immunity. So one does cost-benefit analysis between long-term/recurring lockdown (and resultant impact on public health among other things) vs other measures (and resultant impact).
Also, it's probably a good idea to regard the 'herd immunity was never our goal' backtracking with a skeptical eye. It looks an awful lot like the UK were considering '
is it possible to at least partially utilize it?'
The only 'weird' thing then was/is the incredibly blase attitude towards adopting the contact tracing and aggressive testing done in Singapore/Korea etc. Personally that's the first thing I'd be curious about in terms of being '
afraid that they have checked all the possibilities and have come to that conclusion'. Upon what factor(s) - when Italy is screaming almost literally from the rooftops and anyone knows more data will help guide policy better than no data - the UK and the Scandinavian countries have ruled that out, whether it be for legal reasons or some other. It doesn't seem like it could be due to cost. Is it because internal numbers showed they weren't going to have the beds from the get go anyways, etc.
But from there it can get sticky: you run into possibilities such as maybe they didn't want to test aggressively because they needed as few young people as possible to know they were carriers while the herd immunity was built, etc.