There could have been a new fighter or two but it wouldn’t make sense to not see any TIE Fighters or X-Wings because it’s only been 30 years not 100.
Sorry but this argument is silly. Take a WW2 for example (on which Star Wars space combat was based on), completely new fighters/bombers came out almost every year. And in a war state, it doesn’t take 30 years to withdraw the obsolete models. In pacific theatre, when F4F Wildcat was introduced in 1941, F2A Buffalo was essentially completely replaced from the main units. Same happened to F4F when F6F Hellcat and F4U Corsair were introduced in 1943. Then less than 10 years later (even though war state ended), in Korean War you had completely different type of fighters (jet engines, swept-wing design, etc.).
Also there was never an explanation for why A-Wings suddenly popped up.
It doesn’t need an explanation, at least not to someone who is familiar with air combat history. Just take the F4U introduction that complemented F6F as an example. It added to diversity. Commanders had more choice, i.e. being able to use more specific tools for specific jobs.