That's ludicrous. The start of CD sales decline was after a decade and a half of massive CD sales. The market for new CDs was saturated.
You assertion (and that of record companies) conveniently ignores the fact that through the 90s long out of print albums were re-released on CD. For a great many artists it was the first time their work had been in print in decades. So CD sales in the 90s had an enormous boost of nostalgia sales.
Furthermore sales of new music through the 90s was further buoyed by new genres. Hip hop exploded was a footnote in the 80s and exploded in the 90s. Pop country (Garth Brooks etc) also exploded in popularity.
Every Boomer that wanted a copy of Sgt Pepper owned a copy. If they bought their top dozen albums or some compilations with chart toppers they were basically set for music. When they bought iPods they took that collection of CDs and loaded them onto their iPods.
The volume of music piracy was nothing compared to the size of the Boomer economic bloc.
The piracy assertion also hilariously ignores the changes in media. In the 90s most music buyers in the US didn't have an option to buy singles. Most acts no longer released singles. So a buyer wanting one or two songs had to pay $15+ for a whole album of filler. Much to the glee of record executives.
With the iTunes Music Store debut in 2003 buyers could suddenly buy just the one or two songs they liked for a dollar each. They might ultimately spend $15 but they got 15 songs they actually wanted. They also of course had their existing CD collection to source music from. This again led to saturation, most people don't buy tens of thousands of songs. If you have about 300 pop songs (~4min) you've got a playlist that can play all day without repeating.
After 1999 CD sales started to decline once the nostalgia market got saturated and dropped even more once buyers could buy only the tracks they actually wanted. Most bands are lucky to produce an album worth of good songs over a half dozen separate album releases. The sales left after the nostalgia market dropped off was largely just new music sales.