I think if WordPress admins were allowed to require new users to confirm their email address before they can start using the site would help to mitigate comment spam.
The Discussion settings allow you to choose to moderate comments, so only those with x number of approved comments are published. You can also choose to require an account before commenting.
As a local solution for one of my own sites I’ve created a plugin that puts registering users into a pending table and emails them, only when they respond to that email is the user account created and they still have to log in and change the password.
ClassicPress can be configured to only allow comments from logged in users currently and there are other moderation settings built in. This won’t catch everything but it’s a balance between stopping spam and enabling site vitriol to be relatively easily able to leave comments. That balance point will be in different places on different sites.
I think this proposal is something that should be consider as a plugin, certainly as a starting point anyway,