"If an optic produces better results in excellent seeing than another optic, why would it not produce better results in average or bad seeing? The seeing does not change the optical quality of the lens."
That's the $10k question. The short answer is something close to seeing affecting both apertures equally while the optics differ. A premium optic focuses a distorted wavefront into a 1/10 pv mess, and a lesser optic focuses the same wavefront into a 1/4 pv mess.
It's not the optical quality, which is different in both scopes, that composes the final seeing induced image, it's the messy wavefront coming to focus the same way in both. This is the effect of seeing where the optic is dealing with a disrupted wavefront (seeing) instead of creating an abberant one from a perfect wavefront (quality). At least in my intuition, anyway, if it makes any sense.
It's why we sometimes hear or understand a premium optic cannot fix an already distorted one into something better. It only forms a 1/10 pv messy star image from an already distorted wavefront long before the optic tries to form a good or bad image from it. The poor image is due to the seeing, not the optic.
"If seeing varies from moment to moment that means the seeing is “not excellent” seeing. With excellent seeing there are no jumps, scintillation and no movement."
This is not my experience. There is always movement however slight. Even in the best seeing, which is hardly ever completely stable, there are even better moments. Its during those brief best moments where finest Jovian detail rolls in and out of view.
One night I was checking out Plato's craterlets in great seeing through a 6" aperture. I held at least 6 of them mostly steady in crater form during above average seeing good enough to do so. Then out of nowhere, a 7th smaller crater form rolled into clear view briefly, then faded from view. It reappeared 3 times during the observation of Plato. Even during great seeing, patience pays off.
I've not compared with a certified premium optic, but I have used some good ones. At least as good is defined by whomever.
I think Suiter’s “wobbly stack” is the correct analogy. Everything along that stack can impact the final image: Optical quality, diagonal, focus, eyepieces, etc. etc.
In your example you need too add the aberrations in the optics to those in the seeing. That is what makes the difference. Because you can’t do anything about the seeing, the goal for an observer is an “aberration free” “wobbly stack”.
Think of it this way… two telescopes with the same optical quality but one uses a diagonal that is really dirty and smeared the other diagonal is spotless.
Which telescope will produce the better image? Why? The seeing is the same. The optic is the same.
Because the light path before the eye has been compromised in the scope with the filthy diagonal (a diagonal with aberrations) that scope will deliver a poorer image. Because those aberrations in the diagonal have been "added" to those in the seeing.
Now just change the aberrations in the diagonal to those in the primary optic and it is the same process.
Why do people want good eyepieces? Why do people want their scopes to be thermally stable? Why do people want focusers that allow for exact focus? Etc.
Anything that adds aberrations along Suiter’s “wobbly stack” will worsen the image regardless of seeing.
That is why along with an excellent optic you need an excellent diagonal, perfect focus, excellent eyepieces, thermal stability, etc. etc. etc.
You can’t do anything about the seeing but you can do some of these other things that will improve performance in ALL seeing conditions.