The Y-wing was an outdated piece of junk even by the Battle of Yvain; that's why the Rebels had it at all. The X-wing's proton torpedoes deliver the hurt when necessary (just ask Tarkin or Ysanne Isard), and if you want more than that, well, that's what the B-wings are for... Between them and the A-wing, there is simply no role for Y-wings at any point - except cannon bait!