Idk about you but if Zodiark still being incomplete was the final boss, would've been well boring because we all expected it. Its kind of like when people are mad comic book stories aren't exactly the same in movies. If you went in to a comic movie and everything was the same as the comics, you'd expect everything and know what the outcome would be. No difference here. Most people expected we would either fight both Hydaelyn and Zodiark or just Zodiark at the end, because hes been the big bad all this time, but again he was incomplete and still sundered. Now if the story was different and the rejoinings all actually happened and he was complete, would've been a completely different ball game. Idk I think Endsinger was a good final boss. Because it means in the grand scheme of things even Primal, not even the eldest of them are the most deadly threats out there in the universe. So they can really play around with that.