Their new license doesn't forbid others from offering a managed service. It simply requires the managed provider to pay them for a license. AWS, GCP, etc can still create a managed service.
It is a high risk business model to be entirely dependent on negotiating a license with a single vendor that you are competing against.
For MongoDB there are vendors that will keep offering managed MongoDB. I talked with one that is open-sourcing their deployment as required by the MongoDB license. I suspect that no companies will start a new hosted MongoDB offering based on purchasing a commercial license from MongoDB and we already saw the biggest alternative MongoDB provider merge with MongoDB.
Their new license doesn't forbid others from offering a managed service. It simply requires the managed provider to pay them for a license. AWS, GCP, etc can still create a managed service.