MongoDB switched from AGPL to their own license when they couldn't compete with others offering their software as SaaS, so I don't think the GPL is any kind of protection from this. It's just that the GPL is less popular than alternatives for this type of business model.
That's a very good counter example. Although, I'd imagine, the latest AGPL version will be useful for a long time and any further progress in the code base would also be under AGPL, which would not be under risk of becoming an open core project.
