I definitely think that for older fics, kudos are almost entirely feedback that would not have otherwise been left at all, because people just aren't as likely to comment on older stuff.

Completely agree. I used to have fic posted on AO3 and DW/LJ. Over the span of a few months I got 10 comments tops from DW/LJ on ~old fics (3+ months old)...and probably three or four times that in kudos from AO3 (and a few comments as well). It's really helpful, because I know then that people are still reading and enjoying. That's a great feeling for a writer to have. :)