Personally I'm very happy that the open source world is adopting something derived from Borgmon rather than something derived from its supposed "replacement".
Borgmon may be dead in the eyes of some people, but I know for a fact that it's still the only thing monitoring core and critical systems.
Most of the problem with Borgmon, IMO, is the cruft that has built up over the decade+, and neglect due to the Google pattern of "The new thing that doesn't work, and the old thing that is deprecated.".
The difficulty at Google is that developers are rewarded for writing new and shiny from scratch, rather than fix the old but working systems.
This isn't always a problem, as some good things can come out of starting from scratch. But sometimes they throw out too many of the good ideas, in an attempt to be fancy and new.
It wasn't meant to be commentary on Prometheus(which I quite like) at all :)