I had no idea NetMD and Hi-MD were things. Sony certainly shows determination when it comes to continuing development of their technology (even when it's clear they'll only ever be a niche player).
MD certainly isn't the first niche media format to surprise onlookers with its staying power. Laserdisc lived for many years past the advent of DVD, too.
Sony likely made millions off MiniDisc even if it didn't win in the end.