I mean, there's this argument that while everyone who raids heroic raids is definitely a good player, not everyone who DOESN'T raid heroic raids, or to be even more radical, not everyone who normally raids LFR is a terrible player. To give you a fair example, I never raided a day in my life before LFR, I barely even did dungeons, so by the definition of LFR according to the forum I should technically suck at raiding and be utterly incompetent in normal environments… But I stepped into a guild with a new core team starting normal mode Throne of Thunder aaaand turned out to be one of the best players in that group, so much so that they let me replace one of their tanks that had been in the guild prior to it changing servers in the first place because I was better at my monk than he was at his.
While this kind of looks a lot like a "reasons why LFR ruined WoW" thread, it might be useful to note that having something as outlandish as 60-man raids would probably ruin WoW just as quickly as people claim LFR is. I mean… People have trouble scraping together a 25-man team now. If I throw you a bone here and say "okay Deathwing should have been a 40-man encounter", considering 40-man did exist back in the day, who the hell would ever have beaten Deathwing? What group would find 40 people who were coordinated enough to pull off the mechanics of nowadays, especially considering CRZ wasn't even implemented let alone active in a current content raid?
Oh, wait, that'd be probably nobody, and if nobody ever did it, then why would Blizzard spend money designing it? It'd be a colossal waste of their time and resources if they kept things at the level they were at in original WoW and TBC, when interest in the game was much higher and the people who loved the game had more time on their hands for it. They accomplish more by designing content everyone can see and keeping heroic as the hardest difficulty with the greatest rewards than they would making that content for 1 to 10% of their modern playerbase to see.
The truth is, there could be 40-man raids and big, grand encounters before because the encounters fundamentally weren't that difficult. There was a little coordination required, of course, but it pales in comparison to what coordination is required now, and with the current mechanics of many bosses in current tiers, coordinating 40 people in anything but a watered down mode where mechanics are greatly reduced, a la LFR, would be next to impossible. I highly doubt Blizzard's raiding model would've included ever-more-demanding fights with ever-more-people to manage, that's just counterintuitive.