This is an interesting topic for me because 2 months ago my response would have been very different. At this point I do NOT think there are too many antags, and I believe that while antags can turn away new players, the broken MMR system is more responsible for alot of the rage quitting.
Before "Spacelords" I was very much a 'Story-driven single player' gamer. I never got involved in mmo's or moba's and I avoided most things with pvp or any kind of invasion system. I cant explain why, but I didnt ever want to test myself against people who preferred that kind of gameplay and I believed I would surely lose if I ever was forced by a game to participate in that kind of mode.
Over time though on "Spacelords" that started to change. The more I faced antagonists the more I began to enjoy it, the more I raised the level of my own game. Before I knew it, my teams were winning the battles alot more than we were losing them. Even though the rewards are diminished in matches with antagonists, for me, the sense of satisfaction trumps all the other details.
I had about 5 or 6 matches in a row without an antagonist the other night, I knew exactly what I was getting. I knew my mmr was 60 and that my team was similarly leveled so I knew where the adds would be and I knew they would have 400 hp. I was actually WANTING an antagonist, and like Draco said, if I lost, I wanted them again because I believed things would be different the next time around.
Keep in mind, nearly all of these antagonists will turn around two matches later and be on your squad and prove to be great, supportive teammates. I am better at this game because of the antagonists, and while I remain a 'peaceful antagonist' to earn antagonist affinity, I relish the opportunity to not just compete with the great ones who have helped me raise my game, but to send them with their head down back to Uras for a lecture about their failure.
Fix the MMR to provide a better balance when one side is much higher or lower than the other, but keep the antagonists coming.