The general idea is that since the rebels didn’t have a “military budget” they simply got whatever they could get their hands on, and aesthetics wasn’t exactly a priority either. The Y-Wings f.ex. are canonically old de-commissioned CW-era fighters that had been stripped down to their bare essential even before they stole them. Also the OT is after all 20-ish years post PT, so even if some of the more “modern” boxy designs are only 10 or so years old, that’s plenty of time for them to end up looking old and worn in by a rebellion that literally hides out in caves and has minimal equipment for maintenance. The degree of wear also varies from ship to ship. The Tantive IV and the Calamari cruisers are much more well maintained as they were in the service of their respective militaries before joining the rebellion, but on the other hand something like the Nebulon-B frigate is meant to be stolen imperial ships much much of their outer plating stripped off (similar to the Y-Wings.)
As far as retcons go I think they’ve mostly done a good job of it.
To me, that kind of makes sense, but it falls a part in execution.
The original Star Wars had a very distinct look. Even as a teenager I didn’t realize it was inspired by Flash Gordon until I had it pointed out to me. It felt very much like it’s own thing, and I think that’s great art when you put all your inspirations in a blender and have it end up looking like this unique thing.
The designs of the ships in the PT look like something from a completely different franchise. If they were making a modern but retro looking Flash Gordon or Buck Rogers film, then I’d say they nailed it. If they were doing a film that was meant to evoke the feeling of the 1930/40s sci-fi, nailed it. If they were trying to make audiences feel like this is the same universe that the OT took place in, they failed.
I suppose the 1950s car analogy works, but that’s very specific. Cars from the 2000s don’t look drastically different from those in the 2020s. So if the 1950s are the point of comparison for Star Wars tech, does that mean that star ships had only been in production for 50 years prior to that? Because that’s where auto technology was at in 1950.