@dang A suggestion that occurs to me based on this would be deleting accounts but not comments. Essentially all comments remain but are attributed to “anon_1” or something like that.
We wouldn't reassign every anonymized account the same username, though, because that would make discussions hard to follow when there is more than one such account in the thread. That's especially important because the number of these requests builds up over time, and users never ask for their accounts to be renamed back (though one did!), so such an approach would act as a slow erosion in the threads.