The whole "don't buy it if you don't want it" argument is anti-consumer talk. If I buy something, only to have it superseded shortly, I should be entitled to free upgrade. And in the software industry, it is a known practice to offer discounted upgrade to existing users.If it's so bad you don't have to buy it. But if you bought Tweetie 1 you thought it was worth it. If it was worth it yesterday, it's still worth it today. If you think Tweetie 2 is worth it, buy it, if not, don't buy it! It's that simple.
Yes, Apple's App Store does not have flexible payment arrangement. But I think it is more appropriate for the developers to raise the alarm, not the consumers. After all, you don't expect restaurant patrons to complain to the building owner for raising the restaurant's rent.GO COMPLAIN TO APPLE.
And as I suggested earlier, there is a loophole. Offer the upgrade for free to existing users, but charge a fee for certain new features (via in-app purchase). It's too bad Tweetie 2.0 lacks push notification, as that would be an ideal feature to charge extra.