The only way to accept contributions and then make closed-source modifications is with a CLA; in which case it's the CLA, not the AGPL that you're really complaining about.

ETA: OK, so what if a company start out being AGPL, never accepts any contributions, and then when they become established, stop publishing new code as AGPL and takes everything proprietary? Isn't that just "open-washing", taking advantage of all the community good-will and hype around open source?

I don't think so; consider four possible scenarios:

1. They keep everything proprietary from the beginning.

1a. They become established, making decent money, serving some customer needs. Everything is still proprietary

1b. They fail. Good luck talking their VCs at that point into open-sourcing their code (or even getting it into any kind of shape that anyone could use). All their customers are stuck without any options but to stop using the software.

2. They start by making things AGPL.

2a. They become established, making decent money; eventually they take the product closed-source, doing one final release. Their customers continue to be served, but everything is now proprietary.

2b. They fail. The code is already AGPL, so nothing any of their owners or creditors can do to claw it back. Large companies that have come to depend on their software can take their code and continue to use it and develop it on their own if they want. If there's enough of the right kind of people, a community can form around the releases and the project can live on in a pure open-source form.

2a is better than 1a, because at least there was a time when things were AGPL; the AGPL code can still be forked off and maintained if there's a big enough community.

2b is way better than 1b. In fact, 2b can hopefully make 2a more likely, since it's lower risk for people to build their infrastructure on a start-up.