The constant complaining, not going and the letters, yes. Poor DCA. It took A LOT of flack from people, but it deserved it. But it was bad for DL even before DCA launched. The nineties were terrible for DL, and a lot of it was published in the LA Times, which was embarrassing for Disney. I remember during the 50th anniversary celebration, DL fans booed Eisner when he went up to speak.
DCA wasn't the only one to get talked about, though. There was this new parade called Light Magic that was supposed to debut years ago. Well, annual passholders were invited to see the parade before its debut and they HATED it and talked so much about it... So much that it was cancelled three or four months later. The same thing kinda happened with Mermaid. Before it opened in MK, DL/DCA guests complained of Ariel's hair and said it resembled a soft serve ice cream cone. They were also livid at the fact that the screens that showed Ariel and Flounder and the other showing Ariel turning human looked like CGI work. It didn't take long for Disney to change Ariel's hair and switch the computer animation to more of a hand-drawn animated look. When they took the swing dancing away from Carnation Plaza Gardens to create Princess Fantasy Faire, again, fans complained and created petitions. It's coming back this summer. I remember, when we got our new president from DAK, all the fans were nervous about him and what he was going to do to the parks. I guess someone told him about how ed off DL fans can get and he was seen serving popcorn in Disneyland, riding Alice in Wonderland and showed up at a MiceChat event and showered everyone there with a free gift (Golden Horseshoe Revue soundtrack).
I wouldn't say DL fans have power over TDA decisions and whatnot. But I will say they have some sort if influence that, unfortunately, WDW fans just don't seem to have. It's obvious, too. Just look at the DLR AP privileges and then look at the WDW AP privileges. Like night and day. Just recently, DL invited APs to a brainstorming session, with the promise of food and two hundred dollars. Sadly, I never got my invite lol. A lot of why happens at Disneyland ends up getting written about and published in the LA Times, so that contributes as well.
Even saying all of this, though, I still have no doubt things are going to turn around eventually for WDW. Things are just different there.