I have a limited and narrow perspective on this compared to some of you, due to only being in my late 20s and have only spent time on the amateur scene. So take with a giant salt block. Still, even within a fairly limited time and geography, I have seen noticeable trends taking place in dressage horses.
That said, when I started showing in the mid-1990s, everyone on the amateur scene in Colorado had quarter horses, TBs, Arabs, Appys, paints, etc. and there were very few warmbloods about. There were more in the upper levels but still, quite a few local PSG and GP thoroughbreds and even an app doing GP. And in the lower levels where I was at, virtually no one had warmbloods. Then overnight, it seemed, in the late 1990s and early 2000s everyone and their mother got bitten by the warmblood bug and even at Training and First Level, you were unusual if you didn't have one. All these people I knew who always rode little QHs, Morgans, and Arabs suddenly acquired warmbloods. The downside of this was you had a lot of amateurs struggling with hot, huge moving horses they couldn't handle very well, but the upside of course is that standard of dressage improved and the good riders and good horses were getting better and better.
Then it seemed as if everyone who wanted a warmblood but could not afford one got a draft cross. Okay -- like me, lol (nah, I didn't get her to be a fake warmblood; I got her 'cause I though she was cute). My Shire-TBX is about ten years, or more, older than most other Colorado bred draft-cross "sporthorse" types I've come across. When I got her, she was somewhat unusual. Now horses of her breeding are a dime a dozen and seem to be the poor man's sporthorse.