Developers are mad at Twitter because Twitter gave them a platform to develop on, one that their users enjoyed and made them lots of money, and then yanked it away. Simply telling people to "let twitter go" is ridiculous. Just because you have another idea doesn't mean it will be anywhere close to as lucrative and sustainable as a Twitter client.
Just like the article says, nobody owes anyone in this situation. Everyone was basically in the wrong. However, devs are still going to be mad at twitter. You don't have to feel entitled to something to be annoyed when it's taken away.
And don't forget if it wasn't for third party developers building their own clients, we wouldn't have the official Twitter clients on iOS or Mac.