Correct me if I'm wrong but haven't we SEEN subduction in action via submersibles?
I think you are thinking of the pillow lava forming on the spot at mid ocean ridges. This is certainly part of it but it's not exactly "seeing" it like we see a hurricane pass by.
However the existence of older and older pillow lavas farther and farther from the ridge is an observation and we observe mountains raising as noted above. We observe many things including the actual increase in distance between North America and Europe all of which are "seeing" plate tectonics in action.
This is yet another example where someone is incapable of holding more than one thought in their head at a time and so can't grasp the big picture.