Ackbar would have made far less sense here because it would have made the Star Wars universe feel much smaller and because he would not have been able to die in the battle (like Raddus did). You'd then also have to explain why Ackbar isn't anywhere on Yavin IV during a New Hope, why he's not Echo Base in Empire, and why he's suddenly back in Return of the Jedi.
I highly doubt Ackbar was ever intended to be a character in Rogue One.
I mean, I don't think they were going to use Ackbar either but these are not difficult problems to solve, really. Why wasn't Ackbar on Hoth? Because Hoth didn't have any ships more important than transports, he has better things to do with his time. Why wasn't he at Yavin? He was overseeing the repairs of the surviving fleet. etc etc.
For that matter, none of the characters (with the possible exception of Chirrut) would have been difficult to explain away. Hell, you could easily say Cassian and Jyn were at Endor, had they survived. Or maybe they died with the Bothans. Or whatever.






