They spent too much time trying to keep Ross and Rachel alive over 10 seasons for me to buy that they were going to abandon them.
but Ross and Emily were going to be end game. Then the Emily actress became pregnant and she could no longer travel to the US for the show.
I've seen that theory floated about but I haven't heard a writer or creator say that. I suspect some people are reading too much into the fact that Emily was supposed to last longer than she did.
I bet she was going to be in the whole fifth season. Basically, she'd be the Frasier Crane to Ross's Diane. But then she got pregnant and they wrote her out.