You made some good points, mainly that most developers will/already are taking the risk of being kicked out of the Twitter/Facebook/Etc. platform because that platform already has users.
The way I see it, you as a developer are using what they (Twitter/Facebook/Etc.) built to market your app to their customers that they've spent the time to acquire, it's only fair that they can kick you out of their house whenever they want; that's the way a free market should work, fair or not.
At this point, it sounds like App.net will turn out to be a high-quality network of developers and hackers building interesting stuff, but with no real market or audience. Developers don't build for developers.
Your second point was that $50/year is too much for early adopters:
I agree with you, but to even call anyone who's pledged the $50 'early adopters' is ridiculous at this point. No one even knows what App.net is going to be yet or what it could be, and it's a long ways off being released for early beta testing. What Dalton needs to be doing at this stage is just get people to sign up, for free. From there he can figure out if he'll have enough people to go ahead with it or not.
There's being ambitious, and then there's talking gibberish about something that hasn't even been built yet.