When I was raid lead in vanilla wow, I arranged and lead many many runs on Molten Core, a few on Blackwing Lair, and many in Zul’Gurub and AQ20. When news of The Burning Crusade’s reduction in raid size came out, that the new raids would be 25 and 10, I cheered. My guild thought I was being sarcastic (a reasonable guess), but honestly I was thrilled. I still am. Larger raids are for masochists.
The game indicator of winning in WoW is receiving an epic item. In vanilla WoW, the vast majority of epic items came from 40-man raiding. I’ll grant that there was an epic hunter quest, a few limited crafting bits, some world drops, and a top-level pvp set that only a handful of people could get. But overall, if you wanted a reliable way to get epic gear (and win), you hooked up with a raid guild and started raiding… whether you liked raiding or not.