Copyleft licenses like GPLv3 and AGPL make it slightly harder for tech monopolists like Google and Amazon to take over the project and profit of your work.
And contributors have the security that the project will stay free and will not do the classic MIT switch-and-bait to a commercial license.
Which is to say they can still profit from the work. It's there for them to benefit from using exactly the same as it's there for everyone. They just don't get a better deal or more rights than their own users or a homeless kid in Uganda or anyone else.
If anyone contributed to the project under A/GPL, they cannot switch to a commercial license without removing all contributions first. Because they are not the sole IP owner any more.
But if you contributed under the MIT license, you allow them to re-license as whatever they want.
I thought contributions-relicensing has to do with a CLA, or lack thereof, and not the project license per se. Happy to learn I’m wrong on that point, which sections of A/GPL cover that?
EDIT: having thought about it again, I realized that I was bringing strong assumptions about a CLA being in effect, regardless of the specific license. So, without a CLA in consideration, relicensing is more viable (or more of a danger depending on your specific concerns) with MIT. If a CLA is in play then MIT and A/GPL are likely on equal footing, depending on the jurisdiction.
If you want to contribute to an open-source project, you can just work on your own repo and can work together with other people just like it was a real open-source project.
The good part is, you can pull any improvements from the original repo that they publish under A/GPL, but they can't pull from you if they want to re-license their code later. So the community fork will always be ahead in features and bug fixes compared to the CLA-crippled repo.
And contributors have the security that the project will stay free and will not do the classic MIT switch-and-bait to a commercial license.