All Sadow did was use sith magic to detonate the core of a star. While impressive, it's not really all powerful. Kyp who was barely trained, maybe knight level, could ressurect the sun crusher from the heart of Yavin (or a star for that matter) most likely without Kun's spirit's help as he says later but at the time Kun's spirit helped out a little bit. Revan controlled a star. So on and so forth. I wouldn't be surprised if others could do that, but no one has been that desperate and/or stupid.
Wow, there's a lot of bending of facts here. You like to cite Revan had control of a star, he didn't -- the starforge did and he had control of that.
Guess what? We as humans can already control stars and use their energy, it's called SOLAR PANELS. But we could also fire all of our weapons, nuclear or not, at the Sun and it would do jack; not to mention our Sun is not one of the biggest stars around. The amount of power, force or magic or not, required to detonate the core of a star, even a small star, would be absolutely stupendous.
You're right, Kyp was aided by Kun, and they together did an impressive feat.
As for being desperate and/or stupid, blowing up Stars can certainly be convenient, if you want to completely destroy a planet and don't have a handy dandy Death Star, that's the way to go.
About Spirits: I didn't mean to say that the spirits are as strong as the original, they aren't. Spirits obviously only represent a designated portion of the original power. I just don't think there's any reason to believe they get weaker over time. They're spirits, and some, due to using the dark side to sustain them or whatever, shouldn't grow weaker since they're spirits. The spirits don't age or change appearance at all so they wouldn't get weaker either, IMO.
Obi-Wan's spirit expired, you see Ragnos' spirit needing to be charged by force power from PLANETS just to be able to rise out from his sarcophagus. There's more evidence to the idea that spirits do diminish rather than they remain the same. We don't have all the facts, but off the facts we do now, it's a reasonable deduction.
Simus preserved his head in a jar with sith magic, just like Sadow used the sith magic to detonate the cores of stars. No one past Kun used Sith magic, and with that magic the had different abilities which could help in a fight, yet it's not pure Force they are relying on, rather, it's techniques lost over time.
First off, I'll have to agree with Janus, where does it say no one after Kun used Sith Magic? And where does it say that Sith Magic would not be effective in battle?
Simus FOUGHT Ragnos, and was considered the most powerful FIGHTER of the Sith Empire in his time. I'd assume he'd be able to defend himself without needing to blow up a star. There's no evidence to say that Sith Magic would be useless in battle and is merely a series of techniques. Simus was a Sith Magician and the most powerful of the Sith during his time, that seems to imply that Sith Magic has a way on the battlefield.