Generaly, if the game isnt fun for you anymore, dont play it. It is really that simple.
However your delusion may come from the Vault - I know that after I started participating on the main forums and then on the Vault, I became unhappy about the state of he game. It went away few moths after that and I was enjoing DDO again, but ultimately, RL became more demanding and long story short - I play DDO less and less, but enjoy it when I do. Your experience may differ.
As to adress some of your questions:
After I switched to gaming notebook with SSD, any signs of DCing, connection losting and zoning disapeared. I rarely get a lagg and when I do, its clearly server based. But as I said - I rearely get it. Once an active week maybe? I am no tech expert, maybe the problem is, that DDO became more HW demanding over the years, but getting a decent gaming computer fixes almost every of these issues by my experience.
Pay to win - I dont really give a shit. Actual gamig skill cannot be bought and the rest just means that someone will be better in PvE game.Seriously, why give a shit? Its PvE, not PvP. And if you are playing DDO as PvP, then you are doing it wrong

As long as expansions and paying for them goes... I may be trained by othr mmos too much, but it seems normal to me.
Old content stagnation - I dont ignore any pack, that is fun for me (so I do ignore Threnal) and I dont really feel the way you feel. But it depends on the point of view.
I completely agree that TRying is the only replay value. I enjoy running various and diferent builds trough the TR wheel, changing my game style as much as possible (my assasin is trying to run every quest for devious bonus), but ultimatly, I am taking some time off DDO, so it odesnt start to bore me.
Known issues - there is literally nothing that makes the game unplayable. That said - It pisses me off as well, that so much stuff is simply overlooked, non functioning, left to rot. I am all in favor of skipping one update of content and just do one update of known issues fixing. I literally thing that majority of the gaming populace would love that.
Flavor of the month builds - alway been there, Why play anything else? perphaps you want different past life then the one they offer. Or like me, you dont care about the fact if you build is the single one most powerfull one, but you care more about having fun with it and enjoy it. After all - you really dont need the stromgest build to solo EE.
Yeah devs are incompetent. As far as that iron fist goes (lets call it ham fist instead), thet what Vault is there for.
Hmm.. If you havent made a single good, constant friend/questing buddy, I would say that the problem is on your part. I made several.