Trying to gauge the "success" of SOI won't be a conventional matter, that's for sure. It's almost guaranteed to be one of the lowest selling U2 albums ever, but when it's been given away free a month in advance, it makes it immune from any "U2 sales declining" kind of talk. SOI is a massive promotional tool for the tour and the next album, and to that extent I think it will succeed extraordinarily well.
The funny thing about that whole "U2 sales declining" aspect is that the author failed to go further back.
Looking at sales, yes, ATYCLB>HTDAAB>>NLOTH.
However, NLOTH is still a Platinum selling album in the U.S. That used to be considered a great thing, but apparently when one is U2, it's a disappointment.
But let's go further back. Per Soundscan, Pop sold 1.5M in the U.S. and Zooropa sold 2.3M. These albums were released before illegal downloads and Zooropa came on the heals of AB. So what gives?
Albums are not like movie sequels. Usually, sequels decline a bit in sales, but there are plenty of exceptions. And there are times when the sequel is actually better than the first story, but doesn't generate the sales. But albums are not of the same mold. A big hit song can generate a lot of sales for an album. ATYCLB rode on BD for a LOOOOOOONG time.
As for SoI, clearly this album will not "sell' well based on the fact it was given away. But the follow-up may perform well, especially if U2 can produce that big hit and/or if the tour (or SoI itself) are well received by the public.