It's very difficult to shield the vulnerable if a large number of the healthy population are infected.
This is a key point - how many of the healthy population are going to end up infected, as a result of any particular policy?
If we don't keep this number right down, then that currently small number of under 45s who have died will shoot right up (although not as high as the number of young and middle aged people with underlying conditions). The very small shielded population might be safeish if their carers are also tightly controlled (how?) and regularly tested, but not so much everyone else.
All the talk about risk for individuals from different demographics is a bit of a red herring - there's a huge risk to society and the economy from high infection and sickness rates, even if most of those people eventually recover.
The big danger of working hard to persuade people that only their low individual risk of death matters, is they come out and go out and about, but they are much less cautious about catching/spreading the virus and infection rates shoot up. The economy's recovery is immediately put under threat (and far more ordinary - not just shielded - people die).
We shouldn't exaggerate the individual risk of death to people, but we need to make it clear that it's not the only relevant risk. We're all collectively at high risk (especially economically) if the infection spreads widely. It's only when infection rates are low that businesses, the public sector, the health service and so on can keep going as normally as possible. People won't come out to spend non-essential money and support leisure businesses, for instance, if infection rates in the community are high.
To keep infections low, we need people to care about avoiding infection, not to be thinking "oh well it doesn't really matter so I don't really need to take any precautions". If we're going to improve people's confidence, it needs to come from the government persuading us we're at low risk of catching the disease (not just that if we do we probably won't die).
That can only happen if the infection rate gets very low at this stage, and if when lockdown eases they've got the testing, contact tracing, isolation and so on working extremely well. It's harder work and less appealing to some pundits, but other countries have shown it can be done.