The issue here is that SaaS providers are building tools on top of MongoDB that effectively add functionality, instead of modifying MongoDB (which would force them to share those changes publicly).
This is a valid concern and I understand the motivation behind the license change. However, how is this different from any other FOSS?
Just as a random example, think of a proprietary blog/site provider like Tumblr and Weebly. They are effectively a SaaS provider that introduces tools on top of open-source web servers (like nginx or apache) to make hosting a website much easier. Instead of building the entire model/code of your website, you simply add customizations using a frontend.
Maybe the comparison is not ideal, but my understanding is that all FOSS suffers from this concern, and the industry seems to be doing well enough.