Look, this argument has been done over and over again, but I just had to say you can't compare metal versus plastic miniatures. And you'll never be able to compare mass-produced, pre-painted plastic miniatures with hand crafted and hand-painted metal miniatures.
I used to be a
huge metal mini collector and painter [insert joke about having the "wall of lead" or some joke about the wife that resents the fact that she has no space to put her clothes because of the two closets filled with half painted metal miniatures] for many years. I even went so far as to work for a small metal miniature company to learn the ins and outs of the industry. I got to meet and work with some of the great sculptors and painters in the industry, and I was at the time, a "metal miniature" snob. If it wasn't metal and hand painted, it was crap!
But, life goes on and responsibilities pile up, and I still love to DM with minis- but I no longer have the time to paint up all of my miniatures I need. I also have this annoying habit of needing the exact (or close to as exact as possible) miniature for every encounter. WOTCs DDM line filled that need. Are the minis the best sculpts, or the highest quality paint job? No, but they fill in fine for most encounters, and the success they have had with the line has put plenty of variety out there to collect from. And I still think a fully painted miniature (be it plastic or metal) looks better than a bare metal miniature any day.
I'll also say that I have half the stress of playing with plastic miniatures than I do when my players handle/mishandle my lovingly painted metal miniatures which are much more prone to breaking/chipping in travel or during play.
I won't argue that WOTC is the leader in the miniature market (although I bet they have a good percentage of the market), but I think you can see their influence when other companies have started to venture into pre-painted plastic miniatures (Reaper, Rackham, etc.) and I'll bet that has more to do with WOTCs success than just the increased cost of tin these days.