There aren't enough details... I feel like the author needed to at least make a token effort to support the level of conviction she has that the developers in q

There aren't enough details... I feel like the author needed to at least make a token effort to support the level of conviction she has that the developers in question are so terribly wrong. It sounds like A was buggy (it dies when it receives a particular sort of request that it doesn't understand). Maybe the devs knew about the bug in A? Maybe they decided it was better to make version B work as intended rather than try to throw in a one time hack to be backwards compatible with a broken version that was going to be a thing of the past in... How long does deployment take? Is it even long enough to matter? Maybe the most efficient solution was to allow some of the instances to die during deployment? What kind of application is it? Possibly a few failed requests while you wait for B to fully deploy isn't a big deal. Maybe the client auto-retries quickly enough that the dropped connection is negligible. Maybe there isn't a client at all. The possibilities are pretty much endless given what we know.