I think you're greatly exaggerating their influence. And adding to the genre? Their metal sections are pretty damn by the books in comparison to other metal bands. I can see why you want to pick them because of those bands you mentioned but I mean Opeth actually has made some genuinely great metal and Mastodon right now (at least Crack The Skye) is writing the type of riffs that Dream Theater has the capacity to do but just won't. They're being way more creative with the genre now than Dream Theater has in a very, very long time.
Sure, Dream Theater has become a bit less influential in the past ten years, but just focusing on their decline would be severely downplaying their influence throughout the 90s. I think it's pretty easy to see how Dream Theater impacted metal music. Even if bands like Opeth are more original now (which, honestly, I don't think so-- I have many Opeth albums, and while I like them, they
all sound more or less like the same thing) I really don't see how Opeth are important to metal. They're important, sure, but not to metal. Maybe important to a rather cliquish niche of progressive rock music. Then again, maybe there's not really any real metal scene anymore to be an important part of.
This, I think, lends to the assertion that Dream Theater are the most important metal band of the past twenty years, as Dream Theater are one of the-- if not the--
last important metal bands. Often times, in the world of "art," the last important figures of a particular genre are also some of the most excessive, self-indulgent and controversial. For some, "Watchmen" represents the zenith of what graphic novels could be. For others, it is symbolic of the end of the medium-- it's so overblown that it no longer is in touch with the initial "nature" of what of graphic novel is supposed to be. Another good example is John Milton. For a lot of people, he's the last great epic poet. For later critics, here's an example of where poetry went horribly horribly wrong and became more about style than emotion.
This, for me anyway, perfectly sums up Dream Theater. Because of Dream Theater's technical expertise and theoretical approach to metal, some people would argue that they represent metal in its most perfect form. For others, they represent the point where metal has become so disconnected from what it's supposed to be-- about energy, passion, etc-- that it's killed the genre.
So think what you want about them and their influence, for better or worse. At the end of the day, though, they were the only ones really holding the torch when everything crumbled.