It's not great if you're working on a new release and you realize you also need to change something unrelated because the language changed under you, especially

It's not great if you're working on a new release and you realize you also need to change something unrelated because the language changed under you, especially if it's just a bugfix but a high-priority one, or consider the head-aches caused by source-only distributions suddenly breaking for all your new users (or existing users switching to a new computer or spinning up a fresh VM).