If the add-on is actually causing excessive page requests and erroneous support inquiries, this seems like a very reasonable response. And they didn't ask the users to uninstall the add-on, they notified them of a supplement to the agreement stating that users shouldn't expect support if they wish to continue using it.

Rather than jumping on the bandwagon of accusing tumblr for infringing on user's rights ,I think we should give the guys the benefit of doubt which is well deserved for creating an awesome free platform.

There probably are some serious technical reasons why this add-on is creating problems for them and affecting user experience.

'....its own official “app store” instead of whining about support and server issues.'

No, you can't just willy nilly suggest that each website have their own App Store.

So glad I wasn't alone in that thought.

Whining? Just make an app store? I don't know which is worse the stupid complaint or the absurd solution?

Wouldn't be easier for Tumblr to bring the desired features of the addon to Tumblr?

That won't magically make people stop running the broken addon. If they went so far as to put this dialog box on Tumblr, the addon must be doing some seriously broken shit that causes them real pain, and it must be creating real support requests.