Setting continuity reasons aside, for most of human history, technology improved gradually over generations. It's really only in the past two centuries we've seen the sort of exponential technological growth we're currently accustomed to. The idea of constant progress is a modern conceit. Realistically how much more will we need to develop before we reach a plateau? With Jedi mind powers, cheap unlimited power sources, commonly available hyper-drive routes across an entire galaxy, advanced cybernetics, droids, and lifesaving biotech, it's easy to imagine a society that stagnates for millennia with no real impetus for major leaps in technological advancement.
This. The other thing to look at is improvements over new stuff. For instance, TOR has the hyperdrives, but are they as fast as in the OT? I think an ISD would destroy a Hammerhead easily.
Look at firearms as an example. We are constantly making improvements to firearms, bullets, cartirdiges, etc. At the core however it is fundamentally the same, an explosive charge propel metal along barrel at high velocity to inflict injury upon soft tissues. Some sources cite the Chinese having working firearms as early as 700 AD. That's 1300 years we've been using the same basic principle for firearms, though it has been improved.
The internal combustion engine could serve as another example, sources citing the first ICE to be around the 13th century. That's 7 centuries of improvement, but again pretty similar execution. You can see a lot of these paralells in our tech, where we are constantly "tweaking" it. Star Wars tech for a lot of people would have hit the top so to speak. FTL travel gets the job done so it sits in the "tweaking phase" where people just keep pushing it (like the classic "She'll make .5 past lightspeed". If you look at it that things are getting tweaked, but brand new ground breaking things are invented, then factor in constant strife and it seems the Star Wars universe isn't that far fetched or stangnant after all.