This theory does not take into account that the one of the options is eliminated from the equation.

In the simplified Quiz Show example

Doors 1 2 3

Contestant chooses a door, host shows what’s behind one of the other doors, which instantly eliminates this door from the probability equation.

I am not a mathematician but a wee bit of common sense is all that is required.

This theory cleary shows that even after one of the doors is opened, it is still part of the equation. If the contestant is silly enough to still choose this door then the theory is correct, else, well I think you know the answer