I guess it depends on how important the character is to you -- although at these prices, one assumes all of them are important enough to drop the discretionary income on.
But beyond that it depends on *how much* of an improvement it is. There seems to have been a big jump in paint apps and maybe sculpts as well recently, although the latter is uneven.
How much better can they really get at this scale?
Recently they improved proportions and articulation on Iron Man, and the R1 stormtroopers -- though technically a "new" character rendition, are the closest sculpt I've seen to what we get onscreen. Those are easy upgrades for me. You look at Tarkin and there's not much you could do. Batman Returns? Something to solve the lack of articulation (not that it will ever happen, just an example) would cause me to bite.
As for new versions of characters e.g. Infinity War...it's cool that other people are into them, but outside of *maybe* Black Widow I don't see myself biting on any of the new designs, they don't do it for me because they're not "classic" enough for me.
A true icon for me is Bruce Lee - although it's debatable, I feel like I have the best contemporary release in the EB 75th version, but if someone knocks a new likeness out of the park of course I'll upgrade, it's one of my favourites.
So the answer is -- it depends, but we're in a moment where standards are already very high. It's not like upgrading from old Medicom, SSC or even older HT...the new stuff is so very much on point that I suspect the need to upgrade for quality issues is less, and it's more about a collector's idiosyncratic preferences and renditions of a given character/costume.
Back when there were rumours of an MCU Spidey, I held on to my ASM2 for a while, because I wanted to enjoy a Spidey figure in the moment instead of waiting on some future version, so there is that too.
