> it's simply no longer reasonable to BYO CI
Why? I know plenty of teams which are fine with repo and CI being separate tools as long as there is integration between the 2.
Emphasis on teams; the median open source project has a fraction of a single person working on it.
Actions are bad, but they're free (to start) and just good enough that they're useful to set up something quick and dirty, and tempt you to try and scale it for a little while.