Comment by evanelias

In the "paid SaaS" situation, the SaaS version almost always has proprietary changes and enhancements, which are infeasible to open-source without ruining the business. With the code fully available, since anyone else can then launch an identical business on the same codebase, there's little chance for the project creator's own business to succeed, as it can't stand out on feature-set.

With a CLA on third-party contributions, the project creator can still operate an enhanced SaaS without that massive problem. And revenue from the business can then support sustainable development of the project as a whole.

fwiw, I have recently seen at least two "Show HN" posts where the author was running an enhanced paid SaaS on their AGPL project, which accepted many contributions without a CLA, but did not offer the enhanced SaaS codebase to users. That's a clear copyright violation and any of those contributors can sue the project creator if they wish; this is what I meant by "cannot legally host their own SaaS" although in retrospect I should have clarified the scenario.