MiniDisc came out in '92, well before any portable MP3 players existed. MiniDisc recorders typically had optical digital inputs so you could get your albums on them from CD just fine.
What mattered is whether it was easy to get pirated MP3s from the PC to the player. Fiddling around with optical digital out compared to the much simpler USB seems backwards.
MiniDisks might well have been superior to cassettes but compared to MP3 players? Ugh.
MiniDisks would have been pretty cool (at least for a short time) if you could have plugged them into a USB port.
You could have a handful of disks with 80mins of HQ music on each whereas the MP3 players of the time were either very expensive (like the first harddisk iPod) or had so little space you had to chose a few songs for each trip.
I can understand how MiniDisk could have looked a bit more attractive a year or two earlier but not really in 2001 or 2002.