was there a real question in all that or was it just a huge statement? :)
you seemed to be answering some of your own questions and a few false assumptions ;)
Ahhh ... but we can over years and decades. EVERY earthquake defines plate tectonics in motion. The plate motion is measureable over just 1 year across an active plate boundary region. You DONT need millions of years to see the results.
see the changes for us. GPS satellites gave geologists a whole new insight into tectonics. We can very accurately measure motion and changes of position of fixed objects by GPS triangulation.
Many of the plate boundaries have quite substantial motion eg in the SW Pacific, the Tonga Kermadec Trench area has motions of upwards of 8 cm / year That is very easily measurable with today's technology. This motion builds up substantial stress across the area and when a major quake happens eg the Mw9+ offshore NE Honshu, Japan earlier this year. That stress resulted in a motion of some 20 metres of movement across the fault line.
Back in my old home country of New Zealand, long term motion of a fault line is observable just by going through some farmland paddocks. ~ every 250 - 300 years the Waiarapa Fault in the southern North Island lets rip in a major quake. Offsets for the last 3 events are visible as individual 12 metre offsets with each event. And down on the south coast of the Nth Island you can go back some 5000 years and see multiple events as each event has raised the beach several metres. There is evidence of 6 - 7 major events along the coastline. The last Waiarapa event was in 1855 and was estimated at ~ Mw8.0.
Tho Alfred Wegener did the early proposals of plate tectonics, it took Harold Wellman, of New Zealand, to put it all together with practical observations of what was occurring in New Zealand as he criss-crossed the country doing mapping during the early - mid 1900's.
Others around the world confirmed these ideas with the work done on mapping seafloor magnetic banding anomalies etc. Plate tectonics moved out of the realm of just a theory and into the real world of a practical explanation of observations.