That's fine until you have a 4 MLoC behemoth with 50 developers (I use that term loosely) that isn't separated into logical concerns properly like your average

That's fine until you have a 4 MLoC behemoth with 50 developers (I use that term loosely) that isn't separated into logical concerns properly like your average "enterprise product". I've seen those things and what you end up with is a dev team in tears because some chump has fluffed the source tree, the integration build is still running so no one else knows and checks out the trunk followed by boom and 2 days of paralysis while everyone works all their changes back into it for a checkpoint build. All mature products tend to evolve into this state unfortunately. Ergo, that works on small things but doesn't scale. Minimally, using feature branches and integrating them in turn is the only situation that works on those sorts of products.