It’s not that they can do fine. Any class can do fine. If vanilla is popular and you have a mage wants to tag along and a boomkin that wants to tag alone, you’re picking the mage. Boomkin can only pump out 60% of the DPS a mage can. One big thing to remember is, Vanilla WoW was incredibly imbalanced, even more than it is today.
If we are talking about PvE tier list we have
S - Tier ---- Prot Warrior, Holy Paladin Obviously
A Tier ---- Aff Lock, Fire Mage, Frost Mage, Holy Pal, RShaman, RDruid, Spriest, Ele shaman.
B Tier ---- Demo Lock, Shunter, Prot Paladin, Enhance Shaman, Arcane Mage, Disc PRiest, Destro Lock
C Tier ---- Sub Rogue, Fury Warrior, Feral Druid, MM Hunter, BM Hunter, Balance Druid, Arms Warrior
D Tier ---- Ret Paladin, Combat Rogue, Assassin Rogue
Even in PvP Feral Druids and Balance Druids are c and d tier.