Here we go again indeed, we know enough about the virus and mortality rates,, to make a mature judgement about whether we should still be terrified, or should be living a semi normal life, thats all that bothers non geniuses like me, and the rest of the population.
We
are making mature judgements (despite Boris frequently looking unconvincing

).
We're treading a fine line between getting back to as much normality as possible, and preventing a second wave. You simply do not understand, and have never understood, that if it really kicks off then all hell could break loose. Especially with winter coming, when respiratory diseases always proliferate more freely. This Autumn and beyond is the critical time to be vigilant.
And your persistent assertion that pretty well no-one under 60 dies becomes meaningless when so many people would catch it, that vulnerable people will
inevitably catch it, with the inevitable consequences. We've been through this before: around 50% of the UK population are either over 60 or have underlying health conditions. They live in houses with other people, they work with them, they are 50% of society, you simply cannot say 'make everyone who's vulnerable stay at home'. It's completely impractical and simply wouldn't work.
You're also neglecting to mention the fact that many people who don't die, have long-lasting and sometimes permanent effects which will impact the rest of their lives. Organ damage, a reduction in lung capacity which will never come back.
A vaccine will come at some point and then we can get on top of the virus. But at the moment we're treading on eggshells, and for a very good reason.