First things first: the new license has NO AFFECT on how MongoDB works for its current customers. There will still be a free open source version, a paid support version, and the Atlas web hosted version.
What this new license does is that for people who HOST the open source version of MongoDB and provide it as a service, they have to either get a commercial license from MongoDB, or alternatively they have to completely open source their service. See the license below: section 13 is the added stuff to the existing Gnu GPL V3 license
Chinese cloud companies are trying to host MongoDB free version without providing anything back to the MongoDB community. This move makes it very hard for them to do so - anyone except Mongo who wants to provide MongoDB as a service are going to be required to opensource their entire service.
Mongo is proactively protecting the value of Atlas, and putting up a roadblock to competition, without paying Mongo. It seems pretty reasonable to me, since Mongo is still providing the entire database source code for free. If you want to sell it in a SAAS model, you are required to open source your service, or pay Mongo for what you are profiting from.
so I think this is a smart move, and adding a new layer to open source licenses. It locks in the advantage that the open source developer has, because only they can keep their SAAS code private - all others must open source it, if they use their open source product.
This is the very smart forward thinking on maintaining the moat, that is going to keep MongoDB the top database for decades.
There will be a lot of debates about this in the open source community over the next few weeks or months, but I suspect there will be lots of support for this model, and it will become a new standard type of open source license.