I agree that any leftist policy on criminal justice should be focused on "let's fix the underlying social issues so thoroughly that far fewer crimes happen in the first place." But even in the best possible world I still see horrible crimes happening some.

A lot of abolitionists think this, too, which is why you see "abolitionist" language along the lines of "prison should not be the dominant mode of punishment, but the absolute last resort," but the problem then becomes that's not abolition. Abolishing slavery doesn't mean "we can still do it in select circumstances," it means you don't do it ever. Leftists make that exact point when discussing prison labor and the 13th amendment. It's not coherent to use "abolition" one way when discussing prison labor and another way when discussing imprisonment itself. Also, when people hear "prison abolition" for the first time, they take the term at face value and think we mean it literally, which makes most people dismiss it out of hand ("what about murderers?"). For those still willing to entertain the idea, we're stuck immediately backing off the position we just staked out ("I don't mean abolition abolition"). The phrase is just not a good way of communicating our good policy ideas, and seems to prevent a lot of leftists from developing their thinking on criminal justice in the first place (it's either "prison abolition now" or "gulag!", depending on the facts of a given case).