It's odd to see these takes on whether or not bands took risks and changed. All of this regarding mainstream bands:
Mastodon seemed to take one of the biggest risks of the bands mentioned, going from being a groovy band with catchy, conventional song structures to putting out one heavily influenced by 70s prog rock with many longer, more explorative songs. It's interesting that the risk was to wear old influences on their sleeves, to go towards rock, but to sound more like Yes than modern pop rock that is often blended with their style.
The mention of Machine Head is amusing. Their reinvention was basically to sandwich lousy, overlong, directionless solos between 3-4 minute mallcore songs. The band has always been a D-rate trend follower, from their Pantera worship, hilariously picturesque nu-metal, and now their shitty mallcore groove/thrash stuff. I suppose it's a risk to have four 9-10 minute songs that are basically three different styles mashed together, but it's funny to think of it as a "risk" that the band basically did everything they've done before, stuck haphazardly end-to-end.
Why don't these types of bands take risks? The record industry has become a more exact science in how they're trying to earn money, and one "risk" for many of these bands means they can no longer make a living touring on their music. The record labels don't want bands to shut themselves away and manifest their artistic endeavor over however long it takes. Record labels want to pair a band with one of a handful of metal producers who make everything sound similar, or if they're big enough, a mainstream rock producer. They get a few weeks to churn it out, then they're back to the road to grind it out there.
Look at recent examples. Shadows Fall sold half a million copies of "The War Within" then signed to a major label, took a step towards catchier hard rock, and fell off the corporate wagon. 24k albums sold in the US in the first week was a disappointment when they were trying to market themselves as a pop band. Meanwhile, their comrades Killswitch Engage never took that big leap, they just gradually refined their songwriting in a similar direction while never alienating their core fanbase. They've steadily build themselves to be the premiere band of the style, while Shadows Fall look poised to do the same or more a few albums ago.
The lesson? Even if you're just leaning towards pop, not making unprecedented leaps in extremity, taking risks is going to take a huge toll on your career. Nearly all of the bands on NB/CM/MB/etc follow the same model to try to make a living off of their music, and they need to put out an album every few years, tour the cycles - if they break the cycle, they're going to need to look for a career change.