Wait, if the lightsaber combat skills had been lost, how are Yoda, Mace, Anakin, Obi-Wan, and Qui-Gon able to kick butt?
Since you used Qui-Gon as an example, I assume you misunderstood me. They no longer bothered to train to fight other people who also had lightsabers because it was assumed that everyone with a lightsaber would be on the same side. Qui-Gon was killed by Darth Maul pretty easily once the fight was one on one. Obi-wan only killed Darth Maul because the latter dropped his guard. Obi-Wan started developing his defensive style because of that battle, and he taught it to Anakin and Luke. Yoda spent over 800 years developing his skills with the Force and a lightsaber. Mace took two other Jedi Masters with him to face Palpatine, and they were cut down in seconds.
Wouldn't that make Grievous subject to force powers though? I mean, any force user could just pick him up and slam him in a wall or over a cliff... Also without the force those things won't work as blaster defenses so shouldn't it be possible to finish him with a simple blaster?
Both of those things happened. Mace Windu crushed his torso at the end of the cartoon series, giving him that wheezing cough, and Obi-Wan was finally forced to kill him with a blaster.