It's a cool technology, but the hard thing about building an MMO is architecture, not just making the server faster / wider. If you want a good overview of the types of problems to be solved in scaling an MMO, check out https://youtu.be/-c4t3Y5l5jY.

There is a pretty big underlap between these problems, and the ones solved by SpacetimeDB. So much so that if you built a system using both SpacetimeDB and another, high-level tool to manage chunking, transitioning, server provisioning etc. it would obviously be the latter system that was more indispensable to scaling your MMO.