Richard P. Feynman, the noted science researcher and Nobel Prize winner (1965 - quantum electrodynamics): "The behavior of fluids is in many respects very unexpected and interesting".

It serves no purpose to debate about some of these details when the wind plays tricks on even the best of us in its "unexpected" ways. Those who have been contributing to the discussions found on this website about the electrical details, the Faraday's Laws and all the rest of it, would be surprised to find that a very great level of detail of a similarly logical and compelling nature exists also within the framework of the interactions between the wind and each inch of the lengths along the blades in observing momentum and mass conservation.

The short answer to the objection you raise is that the walls of the tube just simply redirect all the air deflected by the blades back into a straight path again. This also is why wind tunnels have seen little application in wind energy work - unless the facility is very large the walls interfere with the wind turbine's true operation out in the open.