OP here. Why thickness should matter is precisely the question.
If you look at the treatment of this on Wikipedia or a physics text it is the phase difference beteween the front and back reflections which matter and not the total path length.
Even for a windowpane there will be some wavelengths where the two waves are in phase and others where they are out of phase. The true explanation is that for a windowpane these two kinds of wavelenghts are clustered densely together and therefore even for tiny wavelength ranges the interference effect averages out.
For the effect the phase difference is the thing which matters. However, the wave reflected from the back and the wave reflected from the front run through different path length (twice the thickness). For the effect to work the waves need to be in phase along that distance which is a few µm for sunlight (look up coherence length).
The different wavalengths make only up for color. Some wavelength will have destructive interference and go missing. As the eye is quite sensitive to that you will see a colored surface, whichs color changes with view angle (as different wavelengths will cancel for different angles).
If you look at the treatment of this on Wikipedia or a physics text it is the phase difference beteween the front and back reflections which matter and not the total path length.
Even for a windowpane there will be some wavelengths where the two waves are in phase and others where they are out of phase. The true explanation is that for a windowpane these two kinds of wavelenghts are clustered densely together and therefore even for tiny wavelength ranges the interference effect averages out.