I would much rather authors spend more time writing more fic, than more time commenting on comments. Some of my favorite authors never reply to anyone, and I am 100% fine with that, and it doesn't make me less inclined to comment, either.

The tricky thing, I think, is when you want to reply to certain comments, because a dialogue feels appropriate, or in gratitude for some really detailed comments... but then it feels impolite to reply to some but not all.

So I don't have a good answer...