JT had strong sales after 1988 because of R&H (which itself was a big seller) and AB (U2's second best seller). There was no "Best Of" until 1998, so those who liked the songs on JT bought that. Also, in the late 80's/eraly 90's, people were still transitioning to CD. I recall buying my friend the CD of AB in 1991 - she said thanks, but she had no CD player to listen to it!! So people bought music on vinyl and tapes and then, when they got CD players, bought the music again. This is what gave the music industry this BIG push in the late 80's and early 90's. Not only did new music continue to sell well, but catalog sales were huge as music fans re-purchased their music in CD format. No more scratchy vinyl or tapes that break. One could now listen to "With or Without You" 100 consecutive times and the CD would still play perfectly!
Since 1995 or so, those catalog sales were done. Grunge faded and the music industry started to "slump". But did it really? Were people just returning to the way things were before CD's came out?
Also, it really seemed that there was less popular music that was good. Grunge was a blessing. Some hip-hop was a blessing. But there's also been a lot of crap out there since 1995 - some of which peaks momentarily (Spice Girls, boy bands) but fades.
In other words, while JT may be 10x Platinum since 1995, if it were recertified, it might only hit 11x Platinum. 12x if sales were a bit higher over the past decade than I thought. That's still great, but it's not at the pace it was those first 8 years and sales will continue to drop.
Illegal downloading has hurt - I don't think that can be denied. But legal downloading has also changed the way the world works. It's very easy now to get just one or two songs from iTunes. When CD singles came out, people didn't want to spend $4-7 for just "one song". Artists and Music labels tried to entice consumers by adding remixes, live tracks, and some b-sides to the CD's, but at $4-7, consumers felt it was just easier to buy the whole album, not just a single. This, in turn, gave albums a bit of a boost - but CD single sales fell dramatically, to the point where many artists no longer release CD singles. Years ago, 45 rpm records were $1 or less. A person could spend $1, get the song he/she wanted and a flip-side (b-side) and that was great! This is what iTunes has brought back. For $0.99, a person can buy the song he/she wants. No need to spend $4-7 for a CD single or $15 for an entire album if all one wants is that one song.
But by downloading one song, album sales will suffer. Billboard has changed their charting and I'm positive "Vertigo" would have been a Top 10, if not Top 5, hit had Billboard counted iTunes downloads when the song was released. It's a shame that Billboard didn't have this feature available for "Vertigo" - it gives the impression that the song was a "mild" hit, when it was actually rather big. Nonetheless, those downloads would help "Vertigo", but not HTDAAB (unless one downloaded the entire album).
My point is that for any artist to get 2x Platinum is great. 3x Platinum is outstanding. 5x Platinum, like Mariah has, is phenomenal. Illegal downloads and legal downloads of specific songs are decreasing CD single sales. The days of 10x sales, like was saw just 5-7 years ago with Britney and N'Sync, are gone.