Some points:
(A) What do you mean "...anymore"? As you said, the Stones NEVER had been "monster" as album sellers. Never. Their most successful (from the particular point of view- sales) studio album, "Some Girls", sold 9 million copies, at time, late seventies, when Fleetwood Mac were selling 29 m ("Rumours") - and i don't mention Led Zeppelin's or Floyd's incredible sales during 70s...
(
You're talking about the US market. Yes, indeed, ABB sold poorly in the US. We have discussed 1,000 times about the reasons, here. But the whole picture isn't exactly the same. ABB sold 2.6 million copies worldwide. Not a triumph of course, but also not a bad number, considering the fact that the Stones studio albums from 1983 or 1986 (and if i'm right some band's albums from 70s too) had sold 2.3- 2.5 million. At time when the general level of sales was much higher.
(C) Do you think that nowadays, in the years of downloading, ONLY the Stones do lower sales than what once they were managing? Once upon a time a "very succesful" U2's album was selling as "Joshua Tree" did (about 15million) and now the limit is 7-8 m. Sometimes people forget that in the late- 70s and 80s we had sales at the level of 30m, 22m, 25m. Now "zenith" is MUCH LOWER.
(D) Maybe the Stones never had been "monster" as album sellers, but they were and they remain stable enough. That's why they're #12 on the catalogue of best selling acts ever (according RIAA).
(C) Please, name an act born in 60s that sold better than the Stones it's new stuff, worldwide, in last 15-20 years. Please, do the same with acts born in 70s. I think the right answer is "only U2"...