Not withstanding the optical loss there is the problem of holding 1200mm steady. That is 24X a 50mm lens so each tiny vibration is magnifed 24X and it is a bear to try to hold steady even on a heavy duty tripod. What in the world is anyone shooting with such a rig. Perhaps small birds at moderate distances but I think applications for that long a focal length would be very limited.
In the 70's aftermarket 2X and 3X were plentiful and even then, cheap. Mostly they were bought, used one or two times, and then banished to the bottom of the gaget bag. That is why so many old ones look like they haven't been used.