Ace died because he was a barrier to luffy becoming pirate king
he was also a pirate unlike sabo so their paths would have crossed
he was stronger than luffy and he was only going to get stronger
he had a bad ass mentor (white beard) who was there to train him (unlike shanks)
he had a formidable crew (the whitebeard pirates)
white beard beard wanted to make him the pirate king
he had a 2 year head start before luffy
he had the blood of the former pirate king
honestly i cant see luffy ever defeating ace or the whitebeard pirates
and he needed to take down all 4 yonko before becoming pirate king
its like saying luffy would be stronger than sabo
sooo ace had to die and also serve as a powerboost for both luffy and sabo.
Not really sure what the point is in 'theorizing' at this point why Ace had to die, but almost all your reasons can be scratched.
- Ace died because he was a barrier to luffy becoming pirate king
He wasn't a barrier any more than some other characters. There was no reason to kill him for that reason.
- he was also a pirate unlike sabo so their paths would have crossed
Many people Luffy befriended are pirates. Sabo, Ace and Luffy all three started with the intention of becoming a pirate. Sabo simply quickly became a Revolutionary because it was in line with his philosophy: to question the dubious morals and standards of this world and change them. Likewise Ace's purpose was in trying to find the reason of his existence while Luffy simple wanted to be the freest person in the world. So no it was not absolutely necessary for their paths to cross, at least not in a violent way, just because they were all pirates as their believes and intentions were different. There was no reason to kill Ace for that, the fact that he didn't do it with Sabo likewise says enough.
- he was stronger than luffy and he was only going to get stronger
This is a mute point. Many characters were and are stronger than Luffy, but in the end he was able to overcome most of them/will overcome them. No reason to kill Ace for this; Oda didn't do that with many other characters either and Luffy will only grow stronger too you know.
- he had a bad ass mentor (white beard) who was there to train him (unlike shanks)
Luffy had Rayleigh. Yes his training started after Ace died, however Rayleigh didn't train him because Ace died, so even if Ace had lived he would have trained him anyway. No reason to kill Ace for this, in fact I don't even get what you listed this as it has no connection to his death whatsoever.
- he had a formidable crew (the whitebeard pirates)
Luffy has the Strawhats and each Yonkou has a formidable crew. No reason to kill Ace for this and again I don't see why this is even relevant to his death.
- white beard beard wanted to make him the pirate king
And Rayleigh wants to make Luffy the PK and even Shanks' intentions can somewhat be described as going into that direction. Also Ace himself didn't really had any interest in becoming the PK. On top of that many other characters want to become the PK too, do they now all have to die for that? No reason to kill Ace for this.
- he had a 2 year head start before luffy
There are pirates who are an entire lifetime ahead of Luffy. Those two years really don't matter much, especially not the older he gets. There might be an important difference between 16 and 18, however between 32 and 34 it really doesn't matter. No reason to kill Ace for this and how is this relevant to his death?
- he had the blood of the former pirate king
This is actually the only reason that's valid from everything you just said, all the other things can easily be discarded as not even remotely were they forcing Oda to kill Ace. However Ace being the son of Roger has a huge impact on the story. Before we knew Ace's parentage, the idea that Luffy was related to Roger was very credible and following the traditional storytelling, that's what would have been the case with many other authors if they wrote OP. If Ace had not been Roger's son, the most prominent reason to kill him would have been voided.
Frankly for some reason you are way too focused on how powerful other characters are, while the it was obvious from the start of this series that Luffy always transcends above himself whenever he fights a stronger opponent, and use that as a justification for killing characters. "Luffy can never overcome that character, so it has to die", but OP is really the last series this reasoning applies to.
