X-Wings in the EU always got the "they're new" kind of treatment, but I totally agree on Y-Wings... YW's were always a CW era ship that found its way, stripped to the bones, to the Alliance. While the cartoon tried to squeeze them into the CW, the films didn't, and I think Lucas **** the bed on that.
And the ARC-170 is anything but an X-WIng predecessor... Z-95's though? Again, another moment where Lucas could've given a fan wank with that ship, and to the EU, and to Ralph McQuarrie. They too may be sneaking into the CW series, but it's not the same as showing them in the PT...
INstead, 100% new designs like Jedi Fighters and the V-Wing. *shrugs*