TBC did have great sub growth, it was second best however to Vanilla.
In reality the best time that Blizzard ever had was Vanilla and TBC, everything that followed has actually been a massive downgrade in terms of growth. Wrath was just maintaining the status quo even though some will point out peak numbers that are almost meaningless.
The growth numbers speak for them selves, vanilla wow peaked at 8 Million subs, that is just bonkers insane for an mmorpg.
TBC added 3.5 million subs, and retained them.
Wrath added 500 thousand subs, and struggled to retain them.
Cata, MoP, WoD, Legion, and had nice little bumps in numbers, but failed to retain them.
Classic is a very different animal from Vanilla, its got 99% similarity, but the nuance in difference are indeed hurting it and have turned a lot of people away.
- Cross server PVP is a mistake.
- Mega server populations is a misake.
- End game gear stats, talents and spells are a mistake without world, dungeon and raid tuning.
- open transfers are a mistake.
- failing to more strongly police cheating and hacking is a mistake.
Some will point out that we know everything and cheaters have become more sophisticated, and those things are true, but its also true that cheating can be policed even if its tough to do, and the “system” can be enhanced to better derail cheating. Its also possible to tweak, dungeon, raid and world npc numbers, values and mechanics in a way that gives us a more “Vanilla” like experience, because overall the numbers do not matter, what matters is the experience, that is what people were after and it was very foolish to take numeric values literally when everything is far out of range due to the changes made by being out of step with nearly everything character, gear and ability related.