This is a different problem than requiring opt-in to have the mirror started and I fully sympathize. The technical challenge for this is mostly the lack of bandwidth to periodically check for all the posts that ended up being mirrored. To illustrate, alien.top alone already has pushed over 1M comments since it started running a couple of months ago.
It’s not an insurmountable problem, and I think that it’s one that could be solved client-side - e.g, a modified Lemmy client could identify mirror posts and comments and check the original reddit link - or worst-case scenario we could check every day for changes in some percentage of past comments every day to see if there is anything that needs to be updated.