alt Hacker NewsStep 1: discontinue the public repository, step 2: sell access to your GPL codebase.
The GPL (and even the AGPL) doesn't require you to make your modified source code publicly available (Debian explicitly considers licenses with this requirement non-free). The GPL only states you need to provide your customers with source code.
Sure, but it also allows your customers to modify the source code you provided, and distribute/sell it. With MIT they can simply relicence it and sell binary-only versions. The open-ness stops at that point.