Sure the asteroids can be simulated but lets be realistic here. Do we need this many calculations?
We are streaming a demo over what is probably very good internet. I'm pretty sure my crappy 5 mbps internet can't handle the load (and yes I'd like to upgrade it but despite living 20 km from a city of 200,000 in a town of 20,000 in a subdivision of 500+ houses my internet still sucks... **** Canadian telecommunications companies). Many people's internet probably can't handle the load.
This demo is also, well a demo, its optimized for this and designed to do just that.
Again what happens if I start blasting those asteroids? That calculation needs to be done on the xbox and latency is crucial. Streaming the calculations is easy but making them work with real time player interaction is difficult.
And really, 40,000 to 300,000? Does anyone really believe that they are really going to get those kinds of resources for free? I mean if we straight up assume calculations are proportional on the xbox and the servers and that the GFLOP to asteroid ratio is the same that means for every xbox one there is a pair of 7970's in the cloud. I can't see that happening.