Agree 100% You could not be more correct. Most if not all guys that leave the UFC are cut because of poor performance and losses.
I can't remember the last guy besides Hendo that willfully left the UFC. This makes them UFC rejects, otherwise cans in comparison to other MMA fighters.
Andrei arlovski was on a 3 fight win streak when he left the UFC and was ranked #2-3 in the world by all major rankings when he fought Fedor...that was only 14 months ago...
when werdum lost to Dos Santos(who everyone will lose to)he was coming off wins against gabrial gonzaga and brandon Vera (who are both still in the UFC)and was told if he beat dos Santos he would get a title shot....so he losses one fight to someone that everyone is getting beat by and goes from being ranked number 2 in the UFC to becoming a can or reject...yea makes alot of sense...
Jeff monson was 3-1 in the UFC and had just fought for the title and he asked to be released from the UFC they reluctantly let him go....
Tim Sylvia had won 6 out of eight fights(far from poor performances)and signed with affliction....
josh barnett was 4-1 in the UFC but left as a result of positive steroid test...something the current heavyweight champ has managed to avoid thus far...
these are just a few heavyweights I thought of off the top of my head...they were all ranked in the top 10 when they departed the UFC....It wasn't until Sylvia & arlovski faced Fedor that they fell out of the top ten rankings...monson fell out of the top 10 after a lose to josh barnett...and werdum & barnett are still currently ranked in the top ten....