It's my understanding that the only way that Joss could get a Season 5 of "Angel" was if Spike came over to the show. I don't think that "Buffy" ending the previous year had anything to do with that, or at least, I've never heard it mentioned anywhere before. (It would be fun to speculate, though, if Buffy had gone on for another Season, whether Buffy would have ever found out that her two vampire boyfriends were both in LA, and whether any sort of crossover would have been worked out between the two networks)