I don't think you can draw a correlation between the quality of the character's modern ongoing and the price of their silver age 1st appearance. In some instances, the character becoming all but irrelevant certainly has to hurt what was a previously desirable book (like Whiz 2 and Adventure 247), but if the quality of the modern played a huge role, AF 15 wouldn't be the benchmark for the silver age. As you alluded, I think they only real driving factor is movie potential, and in the case of the Flash, the movie isn't in development so there's no real driving force there.
I think a bigger role is the actual value of the book and the significance of the character. Avengers 4, right before the run up, was around $1,000 in 8.0, and that was priced aggressively. SC 22 was around $800 in 4.0 before the run up, AF 15 is the exception to the rule, but even considering the significance of the character, $3K for a 4.0 was reasonable, then boom! I think SC 4 has to have a further pull-back before it can surge, if it surges at all.
That said, I would buy it at today's prices without hesitation because of the certainty that I could sell it at at least those prices in another 3 years, and I don't think there's another top-5 SA key you can say that about and have history on your side.