Interesting stuff Blue, thanks. I should clarify I don't dislike the new canon I just feel there needs to be a justifiable reason why the A wings aren't in wide use pre RotJ.
Maybe they were. There just weren't any at Yavin.
When it comes to the old canon, you need to unlearn what you have learned. It's all starting from scratch again.
Also if it bothers you that A-wings (and B-wings) are not in wide use in ANH and TESB then think about Leia's rebels as a separate cell that used different ships than the ones in Rebels and that the big fleet we see in ROTJ is where all the rebels have finally come together, so all the sudden we see huge Mon Cal ships and new fighters, even though these had been part of the alliance for some time.
This makes a lot of sense actually, although I believe the Mon Calamari hadn't joined the alliance by the time of the battle of Yavin. I'm hoping that eventually in rebels the ghost crew will have something to do with the original theft of the Death Star plans.
That would be cool, but I hope they don't steal any thunder from the Rogue One movie.
Actually in Legend Mon Calamari was officially neutral until 9 months after Yavin but even when they were neutral they slipped the Alliance some ships and crews and other Mon Cal ships which had escaped the initial Imperial invasion had joined the Alliance fleet
At the very least, Ackbar was present and participating in the rebellion at Yavin, perhaps during but certainly immediately after the battle. Take this scene from the Princess Leia comic for example:

This conversation takes place during the evacuation of Yavin after the battle.
The legends timeline is really really complicated if we can both be right. My head hurts now!
At least X-wing PC had the B-wing after Yavin. Rebels will mess that up though I guess.
It's set after Yavin, but in Heir to the Jedi, Luke encounters some pirates flying a mix of fighters, including cloakshape fighters and B Wings. If pirates were using them, I can only assume the B Wing had been in service for some time by then.