Depending on your point of view, the Buffy TV series could've happily ended at the end of season 5. Its only when UPN picked up the show that they milked two more seasons out of it. I love all the Buffy seasons as I'm not too critical about the show.
My point is....whether you think Angel is losing steam or not, the opportunity is there to tie it all up at the end of their 5th season. Whether UPN or someone else offers them another season is up in the air, but there should be enough closure for the fans at least.