Maybe the Patriarch could change his mind once the Divine is free and give the Dragon Knight the power to create new Dragon Knights (without dying).
The reason why I think the Divine will be the playable protagonist of Divinity 3 is for a combination of reasons:
1) In Divinity 2, the whole reason why Zandalor told you to go on the gambit to resurrect Ygerna was because Damian was too powerful to be fought directly. Damian is basically a demigod - the backstory explicitly says that the battles between the Divine and Damian during the great war literally changed geography - flattening mountains and raising plains. That wasn't an exaggeration, look at Aleroth from the Great Market-Lanilor Lane hill - it's REALLY high up now, instead of being by the river like it was in Divine Divinity.
2) The plot of Flames of Vengeance was having Berhlihn tempt the Dragon Knight by offering to free the Divine, and at the end the Divine is freed.
3) I doubt that Larian would go to the trouble of resurrecting the Divine just to have him sit on the sidelines for the final confrontation while a third unknown hero rises up.
4) This game is supposed to be the conclusion of the Divinity arc - which should mean a fight with Damian. Divinity 2 established that the Divine is the best choice in such a fight.
5) The emotional story, the heart and soul of the conflict, is between the Divine and his adopted son Damian. That's the pair that has meaning. Look at how Damian and the Dragon Knight interacted. There was no passion or caring there. Damian was just using the Dragon Knight to open the Hall of Echoes, he didn't care, he didn't even bother knocking down the Battle Tower once the Dragon Knight was incapacitated by Ygerna. He only got really mad when Zandalor told him that Lucian did the right thing by killing Ygerna.
6) Because the emotional conflict is between Damian and the Divine, not Damian and some random new shlub, it would feel wrong, unsatisfying, if someone other than the Divine delivered the final blow to Damian. It would be unsatisfying if the player wasn't in control for that. It would also feel unsatisfying if you build up Random New Shlub #3 for 20-60 hours of gameplay, and then at the final battle, you're not controlling him/her, but a new guy (the Divine) for that.
I think that there was a deliberate purpose in saying that Damian was too powerful to fight directly, and I think there was a deliberate purpose for bringing the Divine back to life, and I think that purpose was Divinity 3.