So I think the question here always is efficiency vs idea constraints. On one hand, you don't want to do double work, so when your prototype is done you don't want to rebuild the parts that are already there.
But on the other hand you don't want to be constrained by writing production code that is fast, readable, multiplatform, etc in the idea phase. So for now we're optimizing for the latter. Until that changes.
So I think the question here always is efficiency vs idea constraints. On one hand, you don't want to do double work, so when your prototype is done you don't want to rebuild the parts that are already there.
But on the other hand you don't want to be constrained by writing production code that is fast, readable, multiplatform, etc in the idea phase. So for now we're optimizing for the latter. Until that changes.