However, The Sith Empire of the old ages was an entire EMPIRE in itself with no republic interference for over thousand years. Sidious' rule lasted for what, less than 30 ?
That's called the EU writers giving into their fanboy urges and trying to expand powers above and beyond to making the original trilogy null and void.
Ironic for a franchise whose fanbase ADORES the original trilogy but HATES the prequel trilogy, which expands upon concepts of the Force and opened the floodgate FOR events like KOTOR and SWTOR to take place.
IRONY! Gotta love it.
EDIT: Oh and BEFORE people start picking apart my response and claim "Tales of the Jedi started that! not the prequels!" and try to pull their butts out of the flames, anything prior to Episode I still clinged firmly to the realm of the original trilogy and was pretty stiff and ridgid by comparison, the most fancy abilities being created through Sith Alchemy or dozens of Jedi binding their powers of the Light Side as one. KOTOR and SWTOR in turn draw from a balanced pool of both the works of Tales and the early EU, and the prequel trilogy.