Solving this is not that complex, really: notify users about changeset comments both by e-mail and by OSM website notification, plus offer an API for third-party applications which would like to offer notification service to their users.
Like many around here, I’m an active Strava user, and by default it spams issues a notification and an e-mail whenever someone comments on your activity, or replies to discussion in the same activity you had commented on, and on many other things. In your user preferences, you can opt-out from types of events and channels of communication you don’t want to receive (e-mail or mobile notification queue). It can’t be that hard. Unfortunately, our front page UI is pretty much stuck in the (next-to-)last decade… (e.g. how hard is to follow a long personal conversation, with messages queued separately in “inbox” and “outbox”…)