From reading all the press, it feels like MongoDB did not do this to garner more revenue but to instead prevent open-source parasites from causing MongoDB to lose money or even go out of business. When I read Eliot's posts in 2018, it feels like he's trying to stay in business, not be greedy.
From a commercial point of view, databases have been shown to need management. Maybe someday somebody will write a database that has a magic simple-to-use endpoint, stays up 100% of the time, and doesn't require a cadre of people dancing around the database chanting spells to operate it. None to date... So the cloud providers capitalize on that need, and wrap control planes around databases - often hurting the people who are working hard to innovate on that software - and deserve to be paid for their work.
As to harm to the open source ecosystem, cloud providers wrapping managed services around databases has been shown to stifle innovation on those open source products. I.e. why would anybody want to improve PostgreSQL or MySQL when Amazon Aurora PostgreSQL and MySQL exist, CloudSQL and Azure SQL all exist - and may or may not take the changes - and even when they do, it takes many months or years.
Finally, I guess I'm puzzled by your last two paragraphs.
You mention that "all the dev hours invested into MongoDB post 2018 likely will have been invested in vain" - why? They go into the community edition, which anybody can use. They go into the EA and Atlas version, which customers who want enterprise-quality software with enterprise-quality support can use.
Your discussion implies MongoDB's license (and Elastic and now a host of others) prevents people from contributing to MongoDB. It doesn't. Or that it prevents them using MongoDB as much as they want. It doesn't, with one exception - and that exception probably affects <0.01% of the users of the software. The MongoDB license (and the Elastic and TimeScaleDB and many other licenses) don't even prevent anybody from wrapping up the software into a cloud service and offering it for money. In fact, many companies (SAP, IBM, Tencent, Alibaba, to name a few, apparently do). It just means you have to talk to the company before doing it.
I personally love MongoDB and I think the SSPL ended up preserving the company and preserving innovation, which is good for the open source community. At least it's not a complicated mess like MySQL and MariaDB.
That said, I'd love to hear more about how SSPL actually damages anybody in a meaningful way. I haven't yet seen anything except a philosophical argument. I'd love to hear it.