runescape is almost the only example there is hence why it gets used.
if we can't use examples it will just be an argument of made up numbers and blizzard saying that they've checked the numbers and data from wherever they got them from.
oldschool runescape was a success even tho their data didn't show it
and for me it seems that the only argument from people that are against legacy servers is that blizzard said so or that it can't possibly be succesfull since it won't get millions of subs.
Many MMO games can run fine with patches and updates without having millions of subs. heck a few hundred thousand is enough to turn a profit and have an active team. and it's not like their main game will dissapear, legacy is a side thing not their main way profit