RDA's aren't dying, rebuildable vaping is, and RDA's are still the most prolific symbol of rebuildable vaping so obviously it's the first thing to do and be noticed gone.
I'd wager a guess that most (if not all) of the tanks in any local vape shop you can find will be pre-built sub-ohm tanks. Most vapers want convenience, and things that offer that include: prebuilt coil heads, a tank that feeds your wicks for you, top filling, etc.
RDA's obviously can't offer that, but a lot of RTA's can't offer all of those things either. Vaping is going toward high wattage, 3-4 battery regulated mods topped with big ass sub ohm tanks like TFV12's and shit that have coils that can handle 250W+. Rebuilding is dying.
Nobody wants RDA's anymore, most people don't want RTA's/RDTA's anymore, nobody wants tube mods anymore, nobody wants unregulated mods period anymore really, nobody wants bare minimum regulated stuff like PWM mods, Hex Ohm's, OKL's, Raptors, etc, nobody wants low wattage regulated stuff anymore.. it's all dying man.
The mass majority of vapers are cloud bros walking around with RX300's, DNA250's and GX350's, topped with TFV12's and shit, blowing vapor in public venues and shit, unknowingly giving us a bad name. I hate to say it, but the average vaper is pretty dumb. The outliers have mostly migrated to bottom-fed devices or a tried and true setup they've owned for years. This 30mm+ shit along with PWM and unregulated stuff is basically a vaping niche that's barely hanging on.
My boyfriend vapes a 22mm Wotofo Conqueror around 100W, I vape 30-41mm shit at 350W+.