I don't think the issue is so much that the backend has to be JavaScript so much as that the backend has to be precisely what Meteor wants it to be. You couldn'

I don't think the issue is so much that the backend has to be JavaScript so much as that the backend has to be precisely what Meteor wants it to be. You couldn't just stick a different JavaScript app conforming to the same interface in the back end — your application has to be more or less monolithic AFAIK.