Irrespective of what happens with cases, if hospitalisations and deaths fall below that of a bad flu season I can see no justification for lockdown.
I see what you mean, but I think the idea is in theory, to prevent another wave, so that the NHS doesn't take another kicking, and so it can be getting back to treating the other people that it needs to, a time of normality as such. We've already had a catastrophic flu season, so we can't get any less than catastrophic without clawing something back with heavy measures. Of course there is potential to make the catastrophe even worse if we derestrict, albeit we've already vaccinated practically everyone at major risk, so although cases could go higher, deaths and admissions probably would not (not like before). It would also be hard to increase cases rapidly with 30m vaccinated, and coming into summer. This seems like a bit of a false argument to me, but it can't be known for certain yet.
For me, we could have a lot less restrictions now, if people were open to having further restrictions in the next few weeks or months, but there seems to be less appetite for this. This is probably a "take advantage whilst it's low scenario", just get it as low as possible, whilst vaccines are being achieved in high numbers, and immunity grows every day. It's probably worse for some businesses opening and closing alternating months, rather than just getting all the shut period out of the way, in one hit. Not my industry mind, some might just rather take any open periods they can get.
I can see advantages for both sides, and I think the government are doing this one as it looks better for them to have one longer lockdown, rather than constant headlines of new lockdowns. They peddle this as "being cautious" to try and make up for not being cautious the other two times, but it's not the case, they're likely just PR managing now.