Re: Galatic stagnation
You said it exactly.
As for: "Major warships are now built to last decades. There's no reason to believe that spaceships will be any different."
I would say that tech in the old republic and this "new" post Empire era would be built to last century's or millennium ('s) - think about it - this is a "long long time ago in a....." - So a super advanced and old society really - Tech may have reached the "accelerating change" peak or flat line on its own bell curve. Think of Moore's law I guess.
So, no tech is not advancing that fast - a big ole war happened, the empire got its butt kicked - funds and tech people are prolly less now.....
I do however look forward to more starwars tech however in the NEXT MOVIES! There could be cool stuff for sure!
Heck - as we saw on Tatooine - on the far reaches of the galaxy tech is old / salvaged and traded over and over......
Would like to see the Falcon re-fitted now for sure - get that next gen of starwars in there!
Oh and as for the storm trooper with the staff that could stop the light saber - we saw those before in the Episode I-III I think - some bad guy had some....