Without getting into the nuts and bolts of recording contracts, advances, pooling, recoupment, etc. etc. AC/DC really has only themselves to blame.
This isn't about piracy so much as it is being bound to archaic contract terms which still emphasize the album.
The basic problem is that they don't have a fractionalized contract (rare) where they'd get an advance for each single. They get an album advance... and they have to pay it back. And AC/DC probably collects large advances at this point in their career so they need to generate significantly larger sales than the old days in order to repay the label, but times and tastes have changed... but so did distribution.
While one song or two was enough to get people to buy the whole album back in the day, customers now have a very convenient option not to (not even accounting for piracy)... but bands in general haven't become tremendously more adept at songwriting when this change came. They still get by on one good hit or two per album.
So, the solution to AC/DC's problem is this: Either tighten your pockets and stop asking for advances out of line with your shrinking place in music, or write consistently good albums that people will actually be compelled to buy all tracks.
As much as I like their music for what it is: basic meat and potatoes rock, I've rarely ever seen a band make a 40 year career out of recycling the same single entendre over and over--every song of theirs is a cheap metaphor for sex, and they all have a relatively similar sound. It's no wonder they feel threatened... The barely talented always fear change, and rightly so. They're the least capable of adapting.
