I believe the Tumbler should at least double in value between 1-2 years post-EOL.
I rediscovered my interest in LEGO in the middle of this year, specifically Technic, after seeing the MkII. I pretty much only was interested in the Technic line. I had no intention of building or collecting any other theme, even Star Wars, despite being a childhood Star Wars nut.
Then I saw the Tumbler, and was blown away by its angular-plated menace.
It's the one set that fired my desire enough to behold it as a 'have-to-have' exception to the rule.
I have no doubt it will will continue to grab peoples' attention as an exceptionally attractive set well beyond EOL, new Batmobiles and cinematic reboots.
There will be a constant supply of adults returning the Lego fold who will be similarly enthralled by this, whether they be specifically Batman fans or not.
Another smaller subset of buyers for this would be adults who are aware of the set having incidentally seen this in the shops or online somewhere, and despite not currently collecting or building LEGO, would have the idle notion of getting this sometime due to its coolness factor. When they discover it's no longer in production, the regret and active searching might spur them into paying over RRP. Granted, it's a small subsection of potential buyers, and one that would probably only participate short term after EOL before prices get too crazy, but my bias believes this set has enough star power to even draw that unlikely demographic more than any other LEGO set currently.
Anyway, breaking out of Technic with the Tumbler, led me to relent and check out other USC sets, and now I find myself down led the LEGO garden path and here today with an attic beginning to become more and more difficult to navigate with having to tetris all these cardboard boxes. lol