If it was proprietary it would not be free - why should they pay then and not now? I am arguing that they would not, just like they don't now - there is no difference. The problem is with selling software - not with Open Source, if it was entirely Open Source - then they would not have to pay and they would use it.
What? If it's proprietary or closed source then the clouds have pay for to legally offer it to their customers. They can't just steal it without paying.
This is extremely common. How do you think Windows, SQL Server, Oracle, Redhat Linux, Spark Enterprise, Cloudera/Hortonworks, and lots of other products are offered by the clouds?
If there's enough demand for proprietary software then the clouds will do a licensing deal directly with the vendor, however MongoDB gave away too much of the product as open-source so there's no reason to pay them. That's the problem. The other issue is that if it was never open-source then nobody might even be using it in the first place, so it's a hard balance.
If it is proprietary they don't have to pay - they can always write their own - just like it happend in this case. If it is economic to write their own now - it would also be economic to write their own when Mongo was entirely proprietary. There is no economic difference in these two situations. But maybe you believe that it was not economy that dictated the rewrite - but rather it was a kind of tantrum - "others can have it for free why can't we!!!???". I don't think it was an emotional decision.
If that was feasible then they would already have SQL Server and Oracle APIs offered using their own tech instead of buying other vendors. Aurora MySQL and PostgreSQL still uses the open-source code layer on the top and just switches out the storage engines, and it's most likely the same here by using parts of actual MongoDB with a different storage engine.
