Something else to consider, DDM has more competition now. While the genre-creating game Mage Knight is no more, the prepainted minis industry has more players now. Wizkids various Clix games are still going strong (minus Mage Knight of course), Reaper has their new line out, Rackam's gotten into the fray, there is WotC's own Heroscape line . . . . probably more I'm not aware of either.
To step things up, I think WotC needs to change distribution patterns. I'd prefer smaller sets released more frequently, maybe interspersed with the larger sets? Maybe some non-random boxed sets like the Drizzt scenario pack? Anyway, WotC does good market research and I'm sure they are on top of it. Besides, there might not really be a DDM slump to worry about, maybe it's just us here that are suffering DDM fatique.
I dig Heroscape's distribution model. Seems to work for that game. I'd also love a somewhat multigenre DDM game. If WotC releases the "new" d20 Modern and d20 Future games (if we are to even get them ever), minis for those games would be great! I have lots of fantasy, but not lots of modern and scifi minis. One of the reasons I like Heroscape!
My only real complaint about DDM is some of the crappy sculpts of recent sets. There are still some good and even amazing sculpts being done, but more than a few just get me scratching my head in puzzlement, "Who approved that piece of trash? My 80s D&D toys look better!"