Let them maintain it. We can take their permissive code and add it to our AGPLv3 projects literally no questions asked, we just have to replicate the copyright notice and license file. Meanwhile they can't take our copyleft code without also copylefting any improvements they distribute. And with AGPLv3, network use is distribution.

It's MIT-licensed, so they don't have to share their changes. That'll make it harder to take their changes.