Responses
It's the best ending to tie up loose ends. Aqua can't live. Because if he's alive, he'd be carrying the sin of killing his father. Which would be a burden to Ruby's future. Also, hopefully he gets reincarnated. So he could live for himself and not for others. Like Goro lived a life his grandma wanted him to have while Aqua lived his life for Ai and Ruby. If he gets another reincarnation, hopefully he gets to live a life for his own self not for someone else again
Honestly, it may be an unpopular opinion but it really is for the best. Just like Kamiki had said, if Aqua had gotten out alive that would've left a smudge on not only him but also Ruby,not to mention that Aqua was prepared to throw his life away multiple times for this. He obviously thought that losing his life was worth making Sarina's and Ruby's better.
All in all, this is still a tragedy, but even more importantly than that it's a martyrdom, moreso one that Aqua embraced gladly.