Honestly, it is a pretty big claim, so it'd require some effort to establish it.
Also, there's degrees of demonstration - do you want to prove to us that you are clearly correct, or do you want to prove to us that you've thought it through enough that we'd see you might have a point, but still disagree? The latter is far easier.
But first, since we don't all have a shared art language, you'd probably need to better establish what esthetic you're talking about - three to five pieces that all share the elements you don't like. Preferably, these would be from different publications, or at least different artists. Then, if you wanted us to not think you're just not happy with one or two recent pieces, you'd want to take several more, from other works. If you want to really establish a trend, you'd need enough for us to feel you've got a solid sample of WotC art, in general, and WotC uses a lot of art...
And, you'd want to be prepared for counter arguments, like noting that a lot of people would feel that art from Dragon Magazine, including, say, Trampier's "Wormy" and Phil Foglio's "What's New with Phil and Dixie" are solidly part of the "classic" aesthetic.
