I think this is a great explanation, I remember when I first saw the movie thinking that Dormammu could eventually break Stephens will, how many times could a person endure dying , it would have to take a toll eventually . With Dormammu existing out of time, I could see him willing to trade a thousand years for the victory.
The fact that he concedes the win to Strange suggests he doesnt think he has another option, it makes sense if Strange is resetting every time, Dormammu would realize if he can endure one death, he can endure every cycle.