Take this with a grain of salt, but I'm fairly sure tech regression is a thing in Star Wars. Lots a tech gets passed around, yeah, but the knowledge to make it does not. I think this got talked about somewhere.

As for the things you mentioned:
- The Mass Shadow Generator was a one time thing, for good reason. It killed almost everyone in the system, including the people that deployed it. Additionally, no one even mentions the possibility a 2nd Generator, meaning it was likely a unique piece of tech that couldn't be replicated.
- I mean, we could possibly wipe the surface of a planet with the WMDs we have today. I'm sure any civilization with star fleets would be able to do the same.
- The Star Forge was one of a kind relic from a forerunner civilization. It was leaps and bounds ahead of anything anyone had ever seen before. Additionally, it ran off the dark side of the force, making it literal magic.

I believe Bioware originally wanted to go hard on the simplicity of the tech of the Old Republic era. Really make it into this dark age where none of the home stays of the series were invented yet. However, Lucas Arts vetoed it since it's a Star Wars game and people expect to see jetpacks and space ships. If the Old Republic era ever got a hard reboot, I'd love to see this grungy medieval age of Star Wars tech of the era.