Plus, dropping a character is not the same thing as firing an actor. Characters run out of steam and get cut all the time. Cordelia had run out of material pretty much in season 2 of Buffy. Keeping her on for so long was too long for most of us, she was fortunate to keep going even into Angel.
This, this, this. If we were talking about Alyson Hannigan, it would be a different story, but Cordelia was a one-note character played by a woman who couldn't act. He didn't owe her anything -- he definitely didn't owe her a prime spot on a spin-off.