Unfortunately, I think if NLOTH had been perceived as a massive success on both fronts (critical AND commercial) then they would have been far more likely to slip out something that would be from the outset expected and understood to be less of a commercial prospect. While No Line was mostly critically loved, and while not selling as well as the last two albums it was certainly no dud, I think the perception is there that the album was soft on both fronts for U2. No idea what U2 think, but there is definitely that perception out there. Really, Boots has a LOT to answer for.
Given that, U2 might not want to see two perceived 'lesser' albums in a row put out. We all applaud them for what they did with Zooropa on the back of Achtung, but if Achtung had not been such a resounding success on all fronts, there's no way they would have done that.
I just think that the initial excitement about SoA from Bono back when No Line was launching would have been due to a high confidence in No Line. Early critical adoration, plus a belief in the songs - that the hits and sales would follow. It didn't quite work out that way. None of the singles caught fire, and there is a bit of a soft perception out there. They would be reading into that too, and I think unfortunately that makes a pressure-free, less commercial, more interesting release less likely. The album will shift closer to being the next Big U2 Album, and further away from being what originally was likely thought of as a more meditative, smaller book end to what was expected to be a BIG/great album on all fronts.