From where I look at it, once you have the endurance base that an IM gives you, going longer is not as big a challenge as going faster.
If they had some IMs that had more aggressive cut-offs (14 hours?) they would inevitably get more prestige value, which is what we are all after at the end of the day. What's the first question you ask someone who has done the Norseman? What colour T-shirt did you get, of course. The fact that even a white t-shirt at Norseman is epic, its the prestige of the black that drives people.
That's not to say I don't want to do some long endurance events again, but is a Double IM harder than an 11 hour IM? I don't know, I haven't done either, and comparing these sorts of things, especially using "hardness" as the measure is pretty much meaningless. But I do know that is I was asked to choose to do one of them I'd choose the double IM as the easier option for me. (2 consecutive IMs, not double swim, double bike etc, that sounds hellishly boring!)
I am an Ironman, but could I ever become a quick olympic distance athlete, or a sub 3 hour marathon runner? I doubt it, because I'm too lazy to push myself into the hurtbox in training and racing. So going long is where I can get my prestige.
Am I a better athlete than a 2.15 Olympic distance athlete? Of course not, but the 'public' might think so because I have done an IM, regardless of the time i took to do it in. The perception of distance being hard is wrong, but long may it continue from my ego's point of view, even though deep down I know speed is where the challenge lies.
Once the public twig, then marathons, IMs, double IMs will fade away and they will understand speed is harder than plodding on.
Good debate though, carry on!