The longevity of immunity is an unknown but I've read and heard suggestions that it should last at least a year and perhaps longer. The wider concern is the implication for any vaccination programme if immunity is relatively short-lived: having to vaccinate 6-7 billion people possibly every 2 years would be both a massive exercise and a signicant cost to the global healthcare budget.
Even 30% of the population being immune means around 20m cases, a 200k-400k death toll and a severe strain on hospitals. Think of London's experience and double it.
I don't think it's fair or even possible to expect all vulnerable people to remain cocooned for another year or more.
But the evidence we have so far shows that Covid 19 has an incredibly small IFR for people under 40, lower in fact than seasonal flu. Why would they need a vaccine? It's an incredibly dangerous virus for the over 75's and of course they should be the priority for vaccination.