Thats BS. The goal is to always make the most money with your music. They did all they could with that style and had to evolve. Money changes people, but their musical style isn't just because of that. Of course the first 2 albums are the best by far, but the others had some bright spots (Three Nil is sick!). Most bands best stuff is when they were poor and hungry, Slipknot is no different.
C'mon man. Corey Taylor literally said himself the reason he got his original band Stone Sour back together is because he wanted to go in a different direction musically and those songs didn't belong on a Slipknot CD. Then your next Slipknot album drops and you have an acoustic song on it....GTFO. Three Nil and The Virus of life were the only decent songs on that Cd. A lot of the songs on the first and second CD were written by the bands original lead singer Anders Colsefni, most people attribute the level of suck that Slipknot has now to that. In my opinion regardless of how good Anders writing skills were, I personally didn't care for his voice.
Of course music is going to change, it is called the music "Business" for a reason. But you don't start off balls to the wall metal, and then become hardrock at your absolute best, without turning a few heads away.
Just as many people were pissed when Metallica got old, stopped drinking, and cut off all they're hair. I see no reason why Slipknot should be any different. Sure those newer albums had a few good songs on them, "Bleeding Me, Fixxer come to mind." but is safe to not argue they never made a truly great cd since the black album. It's all in opinion and taste in the end.