In the original version of Classic,
Ragnaros went undefeated for a whopping 154 days, aka 5 months and some change. It's a very arrogant claim to say it'll be defeated in a week this time around, but unlikely as it is I don't think it's impossible to beat the Vanilla World First record. On one hand, it took lots of preparation for not one person, but 40 people. You then had to learn the mechanics and each boss took multiple tries. That doesn't even mention the fact that attempting this would be the same as if Vanilla WoW players rushed for MC the second the original game launched. Though we do have an infinite wealth of meta-knowledge about the game compared to Vanilla's original launch, we're still limited physically by the game mechanics. For example leveling takes a lot longer than it does in BfA; you can't take a nap and wake up at max level. At best you'd take 2-3 weeks just to hit 20 or 30, so good luck leveling your entire 40-man fucking raid team to 60 before then, and then getting their weapon skills leveled, their professions maxed for the proper pots, food, and gear, and then each of them attuned, as well as farming runs for gear & more mats from the mobs and first few bosses before you can even start in earnest and make real progress.
On the other hand though, that was back in Vanilla when simple concepts of tank-and-spank, CC, and staying-out-of-the-fire was considered hard. Nowadays you'll find that stuff in your average dungeon, while our raid mechanics are drastically more complicated with multiple phases that require several different things from each roll. The skill ceiling is much higher (and always rising), while CC and positioning are just side dish mechanics that are constantly thrown at us with every boss. (though CC in it's purest form isn't used much nowadays, I'm still considering things like off-tanks or groups of DPS focusing on certain minions during a fight to be a form of CC).
TL;DR -
Took roughly 5 months to world first Ragnaros in original Vanilla. Clearing it in 1 week is a
tall order (grind to max lvl, attunement, gear/rep/proff/weap-skill grind, farming runs vs actual raid progress, etc, altogether it takes
months). On bright side, modern raiders have higher skill ceiling than their 2005 counterparts: Positioning, CC, tank/spank
was the challenge for vanilla guilds; for us it's just
the basics.
After required prep work, beating that 5 month world record isn't impossible.