I disagree with you on the fundamentals here. Where do you hear lots of new bands including non-metal influences? The great majority of new bands/new albums are very much strictly metal with very little crossover appeal to rock/whatever fans. Just because some bands are utilizing industrial, folk, shoegaze etc influences doesn't mean it is "more and more"...it might be more obvious because those bands get more publicity for avant-garde or whatever...
And I know it's been mature for a few decades, but we (as humans) have been mature(ish) for about a decade now and yet there's a lot for us to do and look forward to doing despite every day seeming similar in some ways and I think of metal in the same way, with a very human/organic pattern of evolution. In this sense, I think metal is very much "just beginning." Of course it will never be exactly how it used to be and I think this is where you're partially correct, but I believe it's pretty natural.
The sub genres are become full-fledged genres and developing their own sub genres.
Also, now that there's a wide variety of musical categories, lyrical topics are becoming more important. For example, pirate metal and viking metal.
Think about this, though: the defining qualities of each subgenre, the respective essence of extremity of each, their very raisons d'etre, have been amplified/exaggerated by successive generations of bands to the point where they can't be anymore. Think of extremity like a volume knob: you can only twist it to the right so far before you twist the knob off completely and the music becomes a putrid, gelatinous mass that doesn't even make sense anymore. This can be applied to death, black, doom, grind, whatever.
Grindcore can only get so fast (Agoraphobic nosebleed, electronically-inclined goregrind like Lymphatic Phlegm) before it's just an unlistenable blur, before it turns into sheer noise.
Black metal can only be so abrasive (Leviathan, early Darkthrone/Mayhem) before it's just a twisted, hissing mass of, yet again, sheer noise.
Death metal can only get so heavy (Mortician, "brutal" death metal like Disgorge) before the strings literally droop off of the guitars, completely unplayable, the music completely indecipherable.
Finally:
Doom metal can only be played so slowly (early Sunn O))), Skepticism) before it, too, becomes pointless and essentially a caricature of itself.
You see where I'm going with this? Subgenres are almost, if they're not already, at that point. The only thing left to do is a sideways crab walk.
Here's to hoping you're right and I'm wrong.
What I'm saying is that most bands playing truer forms of metal do not represent where metal is going; they represent where metal has been.
post-metal sounds cool. If you could link to the myspaces of a couple bands it would be nice.