It's important to remember that perception of difficulty was different then, which is why classic can seem pretty dull to a modern player. If you know very little about the game, have mediocre tools and varying skills within the group, poor coordination, it'll be much harder. Mechanically it's nothing too interesting, it was just very involved time wise, and consequently much more immersive long-term.

People conjure various reasons for why things are different now, but routinely omit the changes in class talents and gearing.

At launch there was zero spell damage or healing on raid gear as a stat. You might find some green of shadow wrath but items like Robe of volatile power simply did not exist yet.

Warriors, the current top damage class, had a talent rework at BWL release which introduced their main damaging ability at nearly half the effectiveness of where it eventually end up.

Throw in world buff scheduling and the game is very different from how it played at launch.

People in 2006 were really bad overall. We live in a era of min maxing everything.

Players are a lot better than they were two decades ago.

I was playing back then in 2006, you absolutely could. I don't remember any 10-man groups at that time, but people were absolutely doing this with mage-heavy comps with 20 people. And then after Stormcraft Sandbox when all the private servers began to explode in popularity, you had the ability to run back every encounter as much as you wished to speed up the learning curve, test/learn new strats, etc.