AMP keeps users within Google’s domain and diverts traffic away from other websites for the benefit of Google. At a scale of billions of users, this has the effect of further reinforcing Google’s dominance of the Web.
The whole letter is brief and worth reading to determine if you're for or against its message. The key statement:
The Web is not Google, and should not be just Google.
You can sign the letter with a pull request, but you'll have to wait in line.