A restart button seems awfully gamey and you're also relying on the rest of your team having enough of a clue to know the fight is unwinnable and going along with your vote (my last team did not and they blamed me for losing because I went afk while the dragon marched past the last barrier onto the final gate while he had 70-80% hp left)

I'd rather see some sort of progress check where you auto-lose if the boss isn't past a certain hp% when he breaks each barrier, say 90% for the first, 75% for the second, 50% for the third (just throwing the numbers out there but you get the idea)