I argued here Vaccinate the old fast that the old had such higher Covid fatality rates that it was worth trying to vaccinate as many of them as fast as possible as possible.
But what happens to adults 18-50 if they are unvaccinated. The risk of being hospitalised is about 3% across this age group.
The Irish Hospital system went pear shaped in January at '1,846 people with Covid-19 in hospital' which means 50k even if spread out a bit would be really bad for fatality rate of hospitalised cases. Even just for non covid cases if the hospitals are overrun you won't get the other treatment you might need. And we shouldn't traumatise medical workers with the awful conditions that many sick people at once would cause.