Each version of WoW in general has completely different mechanics. Between different versions of the game they have changed effectively everything. If you were to play the live version of the game today and compare it to vanilla, it is much more different in both mechanics and technical implementation than a lot of standalone independent games have been by significant margins. Just think of Doom 1 and 2, or more recently Wolfenstein: The New Order vs The Old Blood, or CoD Black Ops 1 and 2. All those pairs of games reused game assets and engines almost verbatim, with much more similar or identical game mechanics compared to how much Blizzard changes between versions.
The argument these players make is that the version of WoW today is not the game they bought or played in 2006 at all, and by every metric you would measure "difference" in besides "is this the game Blizzard entertainment today calls World of Warcraft?" it would be measurably more different than dozens of games we consider independent products.
The argument these players make is that the version of WoW today is not the game they bought or played in 2006 at all, and by every metric you would measure "difference" in besides "is this the game Blizzard entertainment today calls World of Warcraft?" it would be measurably more different than dozens of games we consider independent products.