[Snowflakian]

Personally, I think MU/MI will stay popular so long as produced. Probably even long after so long as a new better engineered 1:18th scale figure series for marvel isn't created by then.

A key factor on line popularity over long durations has a lot to do with articulation, detail, and scale. There are other variables to consider, but when you look at old school toybiz, much of what has gone dirt cheap is due to the lack of articulation compared to modern standards. The same happened in the line's heyday itself when they started leaning more towards staction/inaction figures.

When a figure is released of a character that is better articulated than its previous, the newer typically becomes the replacement for the older. When a figure got it right the first time, it typically is the one that'll stay in a collection. If it wasn't done right the first time, then it might be replaced for the more articulated version later.

This is the mistake Mattel had made, though is correcting. Mattel with infinite heroes was a step backwards over modern articulation, but the better detailed they get and more articulation they have with the present offerings of that scale, are now becoming higher in demand.

So until Hasbro completely revolutionizes the articulation methods for the figures, they should stay pretty dominant and present as is. The same could be said of any toy line really. One of the main draws for me at least to the new TMNT line was the higher articulation among the turtles. That's the same draw I have to the megabloks minifigures over the iconic Lego ones. Better interaction to scale ratio creates better displayability and playability. These are the ones that stay with us as collectors or even as kids.

As a collector, many of us want to intermingle lines and cross universe displays. The same is true for kids who play with figures as one full world instead of as each to their own brand.

It really varies from property to property, but there are certain mainstays when the initial ones are done right. MU/MI to me ranks around how Transformers Generations/Classics is a mainstay on resale value.(Classics Jetfire to the new Jetfire coming out is another example of this, as is Warbot to Springer, so in other words, if you want any value on your current classics jetfire and you do plan on buying the new one coming out, sell the old now while it still has any value at all.) They'll go up in value over time unless a comparable medium is available that outdoes the previous version of the same character. This is due to brand recognition as well as many other variables.

When it's steps backwards and better is still available, the step backwards ones don't move. This is what we're seeing with spider-man's lines in comparison to MU/MI/Avengers Assemble. We're also seeing this present within Avengers Assemble itself where the more articulate figures are moving faster than the more rudimentary ones.

It's not about the newer or older, it's about which figure is the best representation. Amazing Spider-man 1's super articulated spider-man is still my movie-verse spider-man for that scale because it was done right from the start in comparison to the rest that have been made since. That's the one I'd expect that would go up in value unless a comparable updated version is created eventually. Even then, with the slight costume variations, I expect that figure will still hold value.

There are many factors to consider in this though. So it's a balancing act.