Still, I can't argue against the soundness of a long-focus 6" or 8" newtonian as a planetary scope, and a lot of the arguments I've seen in favor of shorter focal ratios or larger apertures don't take all of the factors into account. With planetary performance, aperture only rules up to a point of diminishing returns.
This is the way I look at it:
- It's not shorter focal ratios or larger apertures, it's shorter focal ratios and larger apertures. All the points you make are important and represent a challenge to the builder.
- It's worth thinking in the fixed focal length, variable focal ratio/aperture paradigm rather than the fixed aperture paradigm. Typically the thinking is "what is the best 8 inch planetary scope?" I am more inclined to ask the question: "Which is the best planetary scope with a 1300mm focal length?"
From an ergonomic standpoint, comfort at the eyepiece, seated rather than standing, focal length is the most important factor.
So, all those disadvantages to a faster scope you mention, they are there and everyone who views the planets through a fast, larger aperture scope is well aware of them. But one should not be choosing between a 8 inch F/8 and an 8 inch F/4 as a planetary scope, I think we all know that the 8 inch F/8 will better.
On the other hand, someone looking at an F/4 is combining that with a larger aperture, in the fixed focal length paradigm, it's an 8 inch F/8, a 10 inch F/6.4, a 12.5 inch F/5 or a 16 inch F/4. In each case, if the seeing supports the decision, the larger aperture will be the better performer.
I have the series of ~50 inch focal length scopes from 6 inch to 12.5 inch. Around here where the seeing is typically quite good, the weather mild, each increase in aperture results in a more detailed planetary views. In other places, other combinations may be optimal.