Ya, Project Spark really isn't a final stop. Even when it was supported, if you got serious about game development, you really had to move on to something professional. Can't live in MS Paint forever. At some point ya have to move to Photoshop, or a clone.
For me that happened to be Unity for now.
It's really the missed opportunity for Microsoft to push C# and the UWP. Using Project Spark Kode, the way Alice.org generates Java code. Oh well, one major corporation's missed opportunity is one for an entrepreneur.