AGPL doesn't prohibit commercial usage. The vendor just need to provide the sources for the service.

The AGPL does not prevent commercial neither as a user nor as a publisher of the software. It even prevents anyone from creating a derivative work from your software and publishing it under a license that prohibits commercial use.

Someone can absolutely fork your software and use it commercially. That's what FOSS is about: You shall not limit what other people can do with the software.

You may however restrict others from putting restrictions on derivative works of your software. That's what the GPL is all about. You enforce reciprocity and ensure that all works based on your software allow the user the essential freedoms of free software.

Yeah that's what I want, corporations to be unable to close my software behind a network and improve it without releasing the improvements.

That is great, but others have other goals. The AGPL doesn't need to be "the default".