The thing I like about PO (and formerly PrimeCuts, or somebody like RPM or ERG) is that you can just download their packaged releases and be pretty confident you've got whatever will get requested. On iDJPool or BPM, you don't have curated/packaged releases, so you have to go manually select and download the songs you need. (I'm a lazy, entitled millennial I suppose!)
The other side of the two-edge sword is all the fluff that never charts from PO/RPM/ERG that you never get around to deleting (at least, I don't). Or the "quick mixes." I never seem to use those. And now my MediaMonkey library reads 229,000 files. 15 versions of every song, and thousands that never got airplay.
Regarding old music and back catalogs -- I once spoke to the copyright owner of the "Murray The K" special from the 60s, which was a TV special that featured live performances and pre-recorded videos of Motown and other performers. Some of the performances, like Up on the Roof by the Drifters, constituted the only video for a given song in that era that appears to exist. I wanted to purchase the master footage to use in my DJ music video library. The concept of DJs using his video was completely foreign to him, so when I pointed out that Xtendamix had one of his videos (Nowhere to Run by Martha and the Vandellas, filmed in a car manufacturing plant), he said he had never heard of Xtendamix and they never contacted him to get the rights to the video (which evidently would be separate from the rights to the song).
Murray the K's "It's What's Happening, Baby" - (https://www.murraythek.com/its-whats-happening-baby.html)
Bottom line -- there's a good chance some of these services don't even know they're breaking the law. (Or they never heard the story about the Platinum Series back in the day...)