Simple enough:
The OP is the best judge of whether the comment is obsolete, and has no particular incentive to be unscrupulous in leaving an old comment - and even if they did, chances are this feature will lead to more obsolete comments being cleaned up than status quo in my estimation
Right now there's no real way for two users to jointly clean up an obsolete comment thread. If I delete mine, I need to let the conversation partner know they can cleanup their half... but then my "notification" comment remains, and there's in general no way out of this loop
If OP doesn't take action in, say, 2 days, can notify mods anyway
It proselytizes itself: if 100 people on meta see this thread, when they flag other comments as obsolete, they are educating the rest of the user base that it's okay to flag comments as obsolete. Plus there is already some badge-incentive regarding flags to help encourage people
It solves the problem we have that it's impossible for an OP to notify a downvoter (who comments) or commenter that the improvement has been made with no weight added to our UI
So yep, sounds like an easy enough win to me.