They ran out of ideas after season 2 so those last 2 seasons actually were boring and awful(Alful?). In the end I think they cheated to get to 100 episodes so they could go into syndication. The NBC boss told the ALF producers they could come back and do a TV movie to wrap things up but he ended up leaving NBC shortly thereafter so ABC ended up letting them do the TV movie, which was a waste of time.
As far as the series finale, yeah, it was a cheap shot to leave it open like that, just shows their desperation to keep their one and only cash cow alive.
They couldn't have ended the show on a happier note? They should have let Alf's friends come back for him again, so Alf would have decided to go with them now, thanked the Tanners for what they had done for him, but it was time for him to return to his own kind.
Yes, I know there were plans for a 5th season with Alf in captivity. But I don't see how that would have been the least enjoyable for anybody to watch anyway.