You have to remember ties were designed to face off against pirates and criminals, who would have z-95s and similar craft which means the fast tie has similar firepower and a speed and manuverability advantage.
The x-wing came as a shock which is when the Empire took the rebellion seriously and we saw the development of the interceptor, avenger and defender.
As for the y-wing it may hurt in a joust but no pilot who wants to live will go head on they'll get behind the slow bomber and stay there piling on the damage while the rebel desperately tries to lose the more agile tie fighter.
