Riley would have almost certainly left anyhow, under about the same circumstances as they were presented in Season 5. He had just plain run his course, and had little if anything more to contribute to the general story line. Some new character would have been introduced, or an old one brought back maybe Faith, if they could have figured out some way to get her out of prison. Things could have worked out pretty much the same with Faith there instead of Spike through Seasons 4 through 7, although I'm almost certain that they would have never have become lovers. I would hate to have seen Faith burn up at the end of Season 7, but, once again, she could have transferred over to "Angel" and substituted for Spike quite well. She would have had to aggravate someone other than Angel there, since they were friends. Connor might have worked for that, if they could have brought him back much earlier and made him a regular through Season 5.