Yeah, there's always something to be pissed off about even if you have it made in the shade.
They've shown that they can write kick ass METAL songs. I think they still could but find it harder to now and choose not too mainly for financial reasons.
Here's my theory:
I think it started with the black album that they just chose to lean more on the rock side in hopes of at least some commercial success and a bit more money. I'm guessing that they did slightly better than just getting by, which is still not nearly enough and were probably sick of the living poor lifestyle (who wouldn't be?).
So when they saw the massive success and money from the black album they needed to continue along that road going even more commercial rock oriented to (hope to) assure more of that good income (and to prove to themselves that they can do it) and they've continued to do so in order to maintain the lifestyle that anyone would want to have.
Otherwise they may have to sell a house and sports car.