It's been a pretty slow spiral in the 20s and eventually the few 29s that play are going to get tired of playing this bracket and either stick with 19s or 39s. The only way I see this bracket being saved is either Blizzard segregates all F2Ps to their own queues (and hopefully the F2Ps return) or the Twinking community voluntarily cans their 29s and only plays their 20s.
Blizzard segregating the F2Ps makes the most sense because it'll give a better introductory experience to new players using the Trial Account for its intended purpose of trying out the game. I don't think this will happen though because Blizzard honestly doesn't seem to be interested in attracting new players to WoW and have pretty much just entered franchise maintenance mode hoping to be able to farm the whales that still play the game. WoW is the Gen-X of video games now and is not really the kind of game that attracts younger players so I think Blizz doesn't see value in expending effort to grow the game and is instead focusing their resources on maintaining the player base they've already got.
As for the Twinking community voluntarily shelving their 29s and playing their 20s instead, it's a fantastic idea on paper but how would you ever enforce it? All it takes is a small number of 29s who refuse too play by the community rules and it falls apart. It would require the formation of basically a community police force who get on their 29s for no other purpose than to curb stomp any 29 that happens into the BGs. If a Horde 29 is tearing up the BGs they log onto their Alliance 29s and keep camping the offending 29, and the same thing is an Alliance 29 is breaking the community rules.
10v10 and 15v15 BGs are definitely not the norm anymore in this bracket and queue times are regularly over an hour. The F2Ps are not going to come back to the game if it means being grossly outmatched. Fighting against a 29 Daddy Twink as a 20 Baby Twink isn't even close to fighting against a 29 Leveler as a 20 Baby Twink. A 29 Leveler is often poorly geared. I don't blame them for that, their goal is to get to End Game as quickly as possible so why would they invest in the time and effort to get geared for PvP in the 20s bracket when in a couple of days the game will have catapulted them into the land of End Game. On top of that the Levelers that do play End Game PvP don't stick around long enough in the 20s bracket and often play by End Game meta which just isn't compatible in the reality of the 20s bracket. Against a 29 Leveler a 20 Baby Twink can be a force of nature but against 29 Daddy Twinks there's not a chance if they're playing DPS. Baby Twink healers can still be useful but healers are always in short supply to begin with.
I have a hard time seeing how this problem gets resolved. Blizzard doesn't care about Twinks and they apparently don't even care about new players and the community lacks the means to effectively police itself. Eventually the Daddy Twinks are going to get tired of waiting over an hour just to get into a BG that is 6v9 and victory is determined simply by which side has the more players in the BG. When that happens this bracket will be effectively extinct, which is a shame.