Chops and I can really only speak to NA probably. But many 20s players rerolled 29 in DF prepatch at the end of SL when 20s were kicked out of the xp enabled queues. These were typically the players that liked to run 8man hunter premades btw (they basically wanted easy shameless wins)
This had a two fold effect in DF and beyond of both driving away dedicated 20s, especially “alternative endgame” folks as well as reducing wargames since the folks who typically organized wargames played 20s and much of the bracket base left their 20s behind.
In fact there were much more wargames happening during SL and previously before the bracket change because the bracket was built around people being level 20.
Well, I don't know much about what the scene is like on NA, so, unlike your buddy Chops, I'm not going to start extrapolating the views I have with regards to the shitstorm that is the EU bracket these days and assume that it would hold for the rest of you. I mean, I would hope it does, I can't imagine you guys being that special to still keep the random BG queues alive.
But to sort of explain you the EU side/version, if you played here, you would automatically know that many players who are playing 20 right now were actually playing 29s themselves particularly during the DF prepatch when BG activity was at its peak since the change. However, 20s were seen at the time a bit more tame (less bursty) compared to 29s, 29s getting fed up of oneshotted by other own instead of oneshotting 20s, and with the wargame initiatives ran by mainly the Russians during the DF expansion, made 20s more appealing. So, in contrast to what you're depicting, a lot of 20s that did reroll 29 actually came back. And the ones that didn't, well, simply got forced out the bracket since they had no community of their own (and still don't, on EU, at least).
Then the wargame kinda began to die down, like I said early in one of the posts here, it became very restrictive and sort of "over-engineered" imo, new players weren't joining/old ones leaving, kept needing to adjust overtime with Blizzard nerfs/buffs.. So, then in TWW, normal pug BGs picked up but here's where players started to realize that 20s was REALLY not any more better anymore because of how bursty the bracket had actually turned out to be. The DF wargames had mostly diguised this but over time, indeed, the 20s bracket has became just as mindnumbingly braindread and pathetic, akin to how 29s were and most likely are. even more: oneshots galore.
And that's really my whole point throughout. I am not arguing whether 29s should get kicked or not; the whole drama seems pointless from either side given how low quality and low activity the general bracket experience is characterized by. My (rhetorical) question was, well, we kicked the 29s out in their fleeting attempt to re-incorporate into the bracket, but why are the queues, if anything, dwindling? And I've answered it already, the question was posed to EU players (given the context of the OP), not to NA players like Chops who felt the need to interrupt and bark either way.