I think that could definitely be a factor, and it's true that I haven't given much weight to that possibility when discussing the ticketing.
If you're referring to Red Zone tickets from 2015 to Red Zone tickets in 2018, I'd agree with that.
And even if you're looking at top ticket prices from 2015 and 2018, I'd agree in theory that they don't seem that much higher. The top price was about $290 in 2015, and it's now $330 in 2018. I'd argue that a $40 increase is a lot for only three years later, but in and of itself, it's not a giant raise. And the $75 price level from the 2015 tour has been increased to an $80 price level, so that's not a huge change either.
I believe the price categories for 2018 have been (with slight variations from venue to venue):
Price Level 1 - $330
Price Level 2 - $175
Price Level 3 - $112
Price Level 4 - $80
Price Level 5 - $50
These are up a little from 2015, but I think the overall idea that each category is about the same is accurate. Price Level 3, for instance, was $112 at Madison Square Garden this year. In 2015, Price Level 3 was $100. So $12 more per ticket isn't nothing, but it's also not a huge increase.
What I think is a big change is which seats are categorized as which price level. The thing that's bothering me isn't really that the price levels have gone up a bit in three years, even though I do believe that they are being raised higher than inflation justifies for that period. But the quieter type of price increase has been the reclassification of seats from lower price categories in 2015 to higher ones in 2018.
To give two examples: I had seats for two of the eight Madison Square Garden shows in 2015 (and was on the floor for the rest). First example: I had a ticket that was priced at $100 that was in a higher row of the upper level, between the main stage and e-stage, with a full view of the screen, just very, very high up. It actually was a pretty decent view, though I felt it was a lot of money to charge to be that far away. That exact same seat for the 2018 tour has been reclassified two price levels up. It's gone from being a $100 seat in 2015 to being a $330 seat in 2018. Second example: I had a ticket that was priced at $75 in 2015 on the lower level behind the stage, with a pretty decent view of the band (albeit from the side and behind) and an angled view of the screen that made it easy to see the half closest to the main stage, but hard to see the end closest to the e-stage. This has also been reclassified two price levels up. It's now a $175 ticket.
I think there have been a lot of increases like that, where the actual ticket prices seem roughly equivalent just looking at the numbers from tour to tour, but are bigger increases than it initially seemed because lots of seat locations have been reclassified.
I also think for the 2015 tour, price levels and what each price level got you was pretty consistent from venue to venue. For the 2018 tour, it appears more dynamically priced. For instance, looking at the newly added Omaha show, in the upper level sections 220, 221 and 222, the lower rows of those sections are priced at $175 each, and the higher rows of those sections are priced at $106. But, if you look at the equivalent sections at Madison Square Garden in New York, the lower rows of the equivalent sections were priced at $330, and the higher rows were $175. This means that fans in New York are being asked to pay twice as much as what fans in Omaha are being asked to pay. I don't think that's right.
Basically, I think you're absolutely right that U2 have toured a ton in the past three years, and that may have decreased some of the demand this time around. But I also think that there were fans who had every intention of going, who logged into TicketMaster, and discovered that the money they were willing to spend no longer got them what it did a mere three years ago. I know that I was planning on trying to get the same seated locations that I had in 2015 for this current swing (I prefer GA but sometimes go with people who can't stand the whole night), and while I was willing to spend $100 for the same upper level seats I had in 2015, I was not willing to spend $330 for those seats this time. I would have sat again in the side/behind stage seats that I had last time, but I was not willing to spend $175 for those seats this time. I think there were a lot of people who would have been willing to spend $175 or even $330 for seats that were truly outstanding, and were turned off to discover that those high prices were no guarantees of actual seat quality.