to me at least the change isn't really about the fact that demo's can't do damage now, it's that the game has become less fun. before demo could play aggressive during stalemates which was fun for the demo and fun if you didn't die like an idiot for the otherside, now the demo can be aggressive with the charge and just never fucking die and there isn't much more frustrating than feeling like you're doing well and doing like 40 damage and that applies to both the new stickies and vs targe with rockets & pipes.
The game has become much more flat as demo just seems to be about spamming pills or using the loose cannon to do more damage easier. the change to stickies itself is kind of lame, it places the sticky launcher into a similar position as the scottish resistance, a seemingly purely defensive sticky launcher, which seems silly because now two weapons inhabit the same role in the game. as a demoman now, you get facerolled if you try to play aggressive without someone babysitting you, who might as well be doing it instead, because you're primary weapon doesn't do enough damage to ward off people from holding w into you, let alone kill them.
the alternates for the sticky launcher are even worse to be in the game as they are even more boring to play against or more boring to play with. the scot res is actually the most boring thing in the game, put down a carpet of sticks and spam pills...yay demoman, you're class is now even more boring than with default! the targe is probably more fun than the other alternatives for the demoman but unless you're a scout that can just strafe out of the charge, it doesn't really feel like there is anything you can do to stop a charge which is especially true with the new tide turner crap, which is silly imo. overall the vanilla targe is the worst of the bunch because the game just doesn't feel rewarding as a soldier or demoman when fighting a targe demo.
the change hinders the offensive part of demoman and more or less leaves the defensive part alone, as demo you can still pill spam, sticky carpet, sticky trap, layer sticks into chokes by shooting them up into the air. meanwhile, demoman can't kill heavies or sentries (let alone wrangled ones) basically raping hl in the ass, and even if you don't like hl, places like gully and snake last are gonna be unbearable once people start running heavies & and engies or maybe even just heavies. the problem isn't that demoman was nerfed, it's that it was done in a fashion that made the game more boring overall imo