His argument seems sound; I guess you may debate only the numbers. I would assign an a priori distribution of celestial objects on near the solar system concent

His argument seems sound; I guess you may debate only the numbers. I would assign an a priori distribution of celestial objects on near the solar system concentrated on the the planetary plane, so this effect may compensate a narrow f.o.v. to some extent. Still, for a narrow enough field of view, you can make the argument that the probability of observation will be too low regardless of the a priori distribution details -- leading to the (possibly absurd) conclusion that many more planets probably exist. I'm just an amateur, but I believe old telescopes had a fairly wide f.o.v.; this is in part because their resolution doesn't even allow too much magnification. I assume they also had very flexible directing so you could survey large patches of the sky if necessary (and in particular you could concentrate around the planetary plane if you're eager for solar system objects).