Here we come with some real numbers, finally!
1st point is: Classic servers are more popular and in demand than ENTIRE games which are very-very popular also, and profitable. Blizz don't even need to develop anything really new to get these subscribers. Most of you are blind by high numbers that WoW always had, but forgot that many, many games around there have a lot less, but still have their niche.
2nd point is: current WoW sub numbers do not represent active players (strange but true). WoW-census, many years being a trustful site to measure population is showing some real numbers here:
789k active players in last 30 days on NA servers total according to
http://warcraftrealms.com/realmstats.php
500k active players in last 30 days on EU servers (excluding Russia, but there is not so much here) total according to
http://warcraftrealms.com/eu_realmstats.php
So, in total we do have around 1,3 mil active players in past month,even if you double them (counting Russia and China), 150k players on one single Nost is pretty big chunk of it, let alone others less known.
3rd point is well known Everquest and Runescape classic servers which came to reality after almost same accident that we have now with Nost.