I was one of those 5% (Inuria - Human Mage - Stormreaver - Foo). The reason there were so few of us wasn’t just because Naxx was hella hard (it was). It’s because they released Naxx 40 like a month before BC dropped. Everyone knew the expansion was coming and that anything we got from Naxx would be made obsolete with the first quest green.
Our guild wanted to see if we could get some of it progressed through before BC hit, and we did. I distinctly remember the four horsemen. Man, that was a fun (and super hard) raid. The trash alone was enough to kick you up the street and back again.
But the reason so few saw it wasn’t because it was hardcore. It’s because most people had stopped doing anything Vanilla and were trying to amass gold and take a breather before Burning Crusade came.
AQ40 was on farm for a lot of guilds. It was around a long time before we left Vanilla behind, and most raiding guilds at least spent some time in there. Inuria the mage is still carrying the Staff of the Qiraji Prophets.
The reason we did Scholo and Strat with ten and Blackrock Spire with 15 is because they weren’t dungeons at the beginning. They were raids. That was changed after MC so that people could gear up for MC casually with the Tier .5 blues that dropped there.
Dungeons WERE more difficult to Zerg, but they didn’t require more skill. They required that you take your time and pull slowly, but the mechanics weren’t anything special. Almost all of it was standard tank and spank with only a few end bosses requiring anyone to pay attention to anything challenging. They were largely gear checks and dps checks…and designed to make you use CC and move slowly through the space.
The dungeons today are much more complex and require a lot more skill to play at endgame level (M0+). Still, the dungeons of Vanilla required teamwork and patience and careful pulls.
But dungeons and raids aside, the larger game of WoW has always been designed to provide content for people who want to raid, people who want to PvP, and people who want to fish and farm for mats to level professions. ALL of those people have always been serviced by WoW. RP, casual grinding, 5-mans, raids, and PvP. Huge range of folks there, and most of them (even a majority of raiders) have always been casuals.