You get houses based on need. Some councils will reassess your need every so many years. Others do not. depends on your contract.
yours may change when you move. I’m on old style contract - tenancy tor life essentially and preserved right to buy. New contracts reassess your need every 5 years - 1 yr initial tenancy with affordability checks followed by 4yr one after. No right to buy. Every assessment period you need is assessed and you’re given financial incentives to move if you under occupy.
I think there is a benefit to downsizing when older, however, these properties need to be of a kind that you can age into. Properties with easy access, wide doorways, wet rooms/walk in bath tubs, grab rails etc and there needs to be enough of them. Retirement properties without support are few and far between so there actually needs to be somewhere to go before people are forcibly moved. Disabled individuals get priority.
the downside to insisting on downsizing is it breaks communities. You need people to put down roots in order to get the benefits community brings, stability, decency etc.
Should people downsize? Yes if they can stay in the area, of the properties exist, if they’re affordable, if a move won’t be detrimental to health.