Open Source allows us to build software greater than any single contributor. Any single contributor can go out of business and the software still has a chance of surviving. Any single contributor can decide to take their work a different direction and other people can still build what they want with it. We've seen this happen so many times, over and over, and the world is in a much better place because that happened.
Closed, source-available, software vendors are the anti-thesis of that. They want others to participate and contribute, but they want to keep all the rights and benefits to themselves. If they go out of business, or just decide to drop development on a piece of software, the software dies. If they decide they don't want a certain feature in that software, you've got no recourse. That's them having the cake.
But if I offer them some code under the same license they hold everyone else to, using the same copyright rules they use against everyone else, they demand I sign over my rights and use a different license. Were MongoDB to accept a patch, under their own SSPL, under fair collaborative conditions, they'd have to actually abide by the intentionally-insane conditions of their SSPL. But that's impossible, so do they stop taking public contributions? Absolutely not. They take contributions, when accompanied by the contributors rights, just fine. This is them eating it too.
This environment actively discourages collaboration. The parasitic demand for copyright assignment deters me from even touching the concept of improving their code. When I have a client with an insurmountable database problem that could be solved by going to the source code, I'm glad when it's a database that has an actually collaborative approach, like Postgres or ClickHouse. In cases of both those databases, I've been able to actually collaborate with the maintainers and improve the software. When a client has an SSPL version of Mongo? I can't even take a look at the source code.
That last client had actually already paid MongoDB Inc. to come in and have a look at the problem. 10K USD later, the consultant gives up and says, "I'm sorry, this will never work." At least the guy was honest, as opposed to the company, which in their report says, "We promise we can bring down your latencies to single-digit milliseconds. Just move to Atlas."
What can the client do? Nothing. What can I do? I have some ideas, if I could modify the source code of the DB to add some acceleration engines specifically for this client. But I can't. Not out of technical limitations, but legal. Because of a hostile license that was itself plagiarized (illegally) from the AGPL.
So the client moves to ClickHouse, and along the way ClickHouse gets some improvements.