Basically: legit users will have a harder time finding legit ones; criminals will still have access to it.

This is just it that you said. What I think more about what is going to happen is that legit users may not know about mixers again except the one users, but new users that have intention of criminality will still know what mixers are and they will still use it. The criminals will still see ways to do their work.
Let not totally ascribed mixers to criminiaties just like the admin mentioned that not all mixers are bad actors, but a situation where one or two of those bad actor mixer can be trace to this forum is what the admin is trying to prevent it long time effect on members and the forum itself.
Those mixers who have a clean record will continue to exist and legit user will continue to use them likewise even the criminals too will also use them, but saying that generally mixer are used only by criminals may be a big mixup between the facts and assumptions.
all these are because of Sinbad which was used by Lazaro. but it's not Sinbad's fault they were picked by the Lazaro group and Lazaro should be their target, not the mixers.
if authorities are up to finding them, they should just follow the trails. killing mixers business will just make their job harder as it will be done in a very secretive group. it is harder for them once mixers do the most innovative ways like using AIs and telegram bots.