Well I remember playing for many years where we would allow each person to choose whether to play a 1ed or 2ed character.
There is no way we could have done that if they hadn't tried to keep some of the compatibility like they did. It was still difficult sometimes, but it worked. At least the AC methodology was still comparable. It's quite hazy to me now, but I vaguely recall that it was a pain to try to figure out what THACO a 1e character should have.
I remember having mixed parties with 1e monk, 2e specialty cleric, 1e cavalier, 2e fighter, etc. All mixed together using monsters from both 1e and 2e in the same encounter, crazy but we did it.