Where have all the Y-wings gone?

Question

There is a significant battle in both The Force Awakens and The Last Jedi that require heavy bombardment on an enemy target.

In The Force Awakens it is the attack on Star Killer base, and in The Last Jedi it is the attack on the Dreadnought-class Star Destroyer.

Both attacks are eventually successful, but one thing I noticed was in both assaults the fighters sent did not seem to have the right type of vehicle to effectively take on their target. The attack on Star Killer base used primarily X-wing fighters whose firepower did not seem heavy enough to penetrate the oscillator, while the MG-100s seemed to be too slow/not maneuverable enough for the attack on the dreadnought.

So my first thought was, why not Y-wings? They had been used to great extent (mostly¹) sucessfully during the battles of Scarif, Yavin IV, and Endor. They are described as an "effective marriage between speed and firepower" which would have made more powerful than the X-wing and faster than the MG-100. It seems like the perfect fight!

So what happened? Was the manufacturer shut down, were other ships upgraded to render the Y-wing ineffective, was it an out-of-universe reason....

Where have all the Y-wings gone?²

_{1 - Yes, I know the Y-wing did not make the killing blows on either Death Star, but they still were the first choice.}

_{2 - To read to this tune}

Answer 1

The Y-Wing was an old and essentially obsolete design. The first Rebels episode of the third season, "Steps Into Shadow" set in 2 BBY, involved stealing Y-Wings that were in the process of being scrapped, and Hera considered them old then. The new trilogy is set 36 years after that.

Answer 2

The original trilogy xwings (T65 X wings) are old and outdated. The new trilogy uses T70 X wings which are newer, more effective fighters. If there is no newer model of Y wing, ordinance equipted T70s are likely more useful in those applications than outdated Y wings. The BS 17 resistance bombers also clearly held enough bombs to take out the dreadnought. I do not believe any other rebel bomber had that capacity.