You know... I agree with a little of this, and a little of that. No question Metallica was great in the beginning. But so are all bands!
We're talking about metallica being great in the beginning...um how many years ago what that??? I know I was about 15 when I had started listening to them... Ummmm I'm 32 now! That's a freaking long time folks. Hell.. I'm not as 'hard' as I used to be either.

Some of the stuff I used to listen to back then, whooo...it'd give me a headache now.

But the point is...
Like people change with years... our styles change, our tastes change, our entire outlook changes. God, back then I actually liked "SLAYER"!!! You couldn't pay me to listen to that. Metallica...even though they are mainstream these days, I do still like them. Sell out? Maybe...to a point. But if they stayed as they were, they would never have survived. There has to be change...or there's nothing.
Sorry I still love Metallica.
Look at Ozzy in the beginning... and look at Ozzy in his later albums. MAJOR CHANGE! Majorly softened! However... Ozzy is still the masta!

He's not a sellout...he just changed.
Hell look at Manson... the first time I heard his stuff, I thought "What the ...." Thought I should call a priest to bless my house after that (Not really but you get my point) Now he's mellow compared to his first album. They all change.
As for them touring with Limp Bizkit and Linkin Park...
What's wrong with that? I think that would be a kick ass show. Limp, has a couple of decent tunes...and I really like a couple of Linkin's tunes...