For example, if an org wants to modify an open source project in order to adapt it to their specific requirements, having access to a high quality test suite is going to be very valuable.
Additionally, since someone is paying for the tests, it would incentivize the open source developer to create higher quality tests, which in turn leads to higher quality software. It's a positive feedback loop!
Anybody can still use the software without having access to the test suite, and will still benefit from the existence of the tests since stable releases will have been tested.
There could also be different test tiers. Like some core functionality tests are free, but integration tests with important third party services/libs require a subscription. This could be different from a regular support contract, since it'd be more like a typical software licensing situation. Just pay $X/mo for access to the test repo, a CI/CD service, etc with no other commitments from the developer(s).
Is there a flaw in this idea? Would you pay for (or convince your employer to pay for) a test suite?
I've never worked with a team so I really can't say if they experience the same thing. I suppose they should uncover bugs better than I alone but users would still find some.
It took me a pretty long time to suss out all the ways users found to break the app after adding new features so I have to think teams could benefit from a service like this.
Hopefully others will chime in here for you.