I don't really have much to say regarding the actual level of technology.
But there was a similar argument I witnessed regarding the technology level in The Lord of the Rings. Remember, it's a fictional galaxy - it doesn't have to follow real world norms. It doesn't have to be believable, after all. It's all about suspension of belief
And with regards to lightsaber technology not changing for four millennia - there's only so much you do to improve a weapon made of pure energy that cut through pretty much anything