A genuinely unpopular one and I agree with you!

With a partial caveat on your last claim, which seems a bit shaky. Sometimes it's genuinely necessary to lock people up in order to protect society. Not very often, but sometimes (and that not mean treating them poorly).

IMO there's also a case for using prison as symbolic justice. A month here, 2 weeks there, to send a firm message about what's acceptable in society. But the rule should be community service, not prison.