[crazygringo]

> But if you're singing a long passage without accompaniment, without absolute pitch you may end up drifting.

You don't need absolute pitch to not drift.

You just need to maintain relative pitch. That maintenance requires attention and some practice, but it doesn't even remotely require absolute pitch.

And remember that singers with absolute pitch have their own problems, in that singing the same song when accompanied by a new piano that is tuned slightly differently becomes much harder and annoying. Whereas for most people they don't notice and sing in tune with it effortlessly.

Also, people with relative pitch certainly experience key changes, the sense of tension introduced by a new one, and the sense of "returning home" and relief when you go back. Composers write for that sensation which is shared by everyone. Not for any kind of absolute perception. And I don't know why you think classical composers overwhelmingly had absolute pitch. Do you have any evidence?