I think there are a lot of reasons for people to not change their mentality about 3rd party with C2:
First, some plugins just are not worth the time for some of them (I know I was personally only rarely interested in 3rd party plugins, most of them just are not for me, only the fgl plugin, the paster plugin, the clay.io plugin, and the spriter plugin caught my interest, from all the plugins I have saw)
Then you have the support problem, not only the plugin might not work anymore, but also you have to keep it up to date, and also, if you have a sort of advanced problem that needs a capx sharing to be solved quicker, a paid plugin will not help at all (and since most people asks for a capx, you see the issue, even just having the moveto behavior can lessen the amount of help by quite a lot).
I understand C2 relies on 3rd party and the community (every exporter other than html5, the SDK, the whole tutorial section of the website), however would anyone buy a plugin with no warranty that it will work in the long term, and so that they can make back their investment either in money or just satisfaction? Maybe, there are some 3rd party plugin that can be purchased after all, and some might say "we should not wait until scirra make them", I feel like it is more "it is hard to think clearly about what to do with them before having them".
For exemple, I do not know, and was not aware of (or do not remember), the multiplayer plugin you mentionned, when a feature is added to the update of C2, and that you can play around a little with it, you can start to feel it and imagine with it, without however, it is hard to concretely base something with a plugin name.
I could be wrong and the truth could be it is a "we want everything free! And officially supported hurr durr", but that seems kind of easy to be the whole truth.