I'm saying that the productivity gain would have to be incredibly large since it has to encompass a doubled output of features and bug fixes for a net zero change.

Say you have product A with features P, Q, R, and S; writing product B has to reproduce P, Q, R, and S, plus X and Y that is currently produced by the A team. On top of that, it has to fix (conceptual) bugs within P, Q, R, and S. All this is to be done by the new, crack, team that aims to make it so that: cost-of-development(B) < cost-of-development(A).

But the point is that the difference in magnitude of cost-of-development(B) and cost-of-development(A) has to be rather large considering the amount of work needed to have a return on that investment at all.