Well, concerning GA to be a boon for younger fans...uh, that's really only the case in these select markets where GA tickets (absurdly) are still unsold. And when we're getting into the arenas, it's basically a joke because it was basically just fan club members that were able to get GA tickets for I&E since there weren't that many. So, having the GA price lower doesn't really mean jack for the younger/casual audience when it almost always entails that you have a fan club membership nowadays in order to snag them.
Those on the floor are generally much older than those in the seats. Plenty of young people in their 20s and 30s in the nosebleeds...the floor is loaded with decades-long fans in their 40s and up.
I do agree that demand is clearly way down for them as a touring act. I&E not having any hits and being regarded as generally shitty by the public and critics certainly didn't help much either. The other big problem is that U2 isn't quite a boomer act like, say, The Rolling Stones or Paul McCartney. That audience wants to see maybe six acts life (often over-and-over) and is willing to spend an exorbitant amount to continue watching them. U2's appeal is a bit broader, certainly far younger, and they can't charge as much as those acts or the "I'm a trophy wife, so buy me tickets" acts (Beyonce, Madonna).
360 probably benefited quite a bit to them not quite playing to demand on the Vertigo Tour, specifically in the United States. So a lot of people that wanted to see them, but really couldn't wanted to catch U2 live and check it off their bucket list whereas Vertigo was the last time they were really relevant critically/commercially. Keep in mind that the Vertigo Tour would have near instant arena sell-outs almost everywhere whereas tickets for a lot of shows on I&E just sat their for ages (partially due to the price) while the cheap seats all sat on resale websites up until the night of.
It would be interesting to see where the demand would be with Vertigo-level prices (adjusted for inflation) in an arena setting and methods to keep scalper bots away...I actually think the demand is a lot higher than you'd expect, but on Vertigo they were charging like $150 total for the lower seats and now they're asking like $300-plus fees...those prices definitely shoo people away.