I don't think anything will ever top Super Powers simply because there are so few of them. I do think JLU and DCUC will go for some pretty high prices one day but nothing like Super Powers (and this coming from someone who doesn't like those). It's easier to collect a line that had 30 something figures, instead of 200.
I doubt either line will top SP either (which honestly I do not think is that expensive anyway) but the reason is because neither line really has the kids now. if JLU had a large children following though I would expect it would surpass SP but I don't think it really did plus alot more people actually keep figures mint and MOC these days so that is working against it too. SP is more rare as you said and also hard to find in good condition because most people played with their toys then.