That's not fair...

1) Costs were not zero for the re-release. The only version of the game data (stats, items, enemy spawns etc.) was in the form of an original database backup (from an old employee's personal stash!). Classic runs on the modern WOW engine, so work was required to shoehorn the old data in and reimplement systems and interfaces which don't exist in the current WOW engine.

2) Before Classic's release, by far the most vocal crowd making demands of Blizzard were shouting their slogan "NO CHANGES". I really don't find it surprising that Blizzard has not made major changes since the majority of the player base requested as such...

Blizzard did not charge for WoW Classic. It merely requires an active subscription to the regular game. There's no additional purchase or subscription.

The company invested significant development into Classic. The project started as a fork of Legion, in order to benefit from a decade of anti-bot measures, compatibility fixes, and Battle.net integration (auth and chat). They then ported the original game forward and added "layering" to avoid crashes that plagued the original game in 2004 and 2005.

If you think wow classic had "practically zero development cost", you don't know what you're talking about.

Blizzard shut a "wow classic" style private server down because they are the owners of WoW but then they left lots of people who wanted the classic experience stranded and decided to give them an official way. It's not a money grab.