I 've seen those so called 10k players per server and in reallity it just does not work and it's pretty much a lag fest. You should see how those servers run in China ( where private wow server are very popular ).
I don't understand how they are able to replicate all the interaction rules. There's a bunch of things going on in WoW, and unless you did a heck of a lot of experiments how would you uncover the rules governing all the interactions between different things? For instance you can bubble hearth on classic but not in hardcore. How is an external server developer going to know that?
Also how do they place all the NPCs and hook up the quests and lot tables? Do they scrape wowhead?
Yes they scrape as much as they can, but the originals of these (almost as old as WoW itself) were absolutely not 1:1 behaviour. By hardcore players noticing the minute differences, taking the time to detailed bug report, then the admins/devs looking with care, they've come pretty close.
Never underestimate a group of nerds with a passion.
the size of 'big engagements' that are still playable has gone from a few hundred to a few thousand in this timeframe. thou players tend to hit the ceilings and "playable" is a matter of opinion.
Tangent: Imho, the only reason it is good is because it's not as grindy and / or the community just didn't put as much emphasis on min-maxing things. GearScore was a thing of course, but theory crafting wasn't anywhere close to what we have now.
Not sure that's true for me, very good raids (minus the trial). Ulduar and Icecrown being the highlights for my guild. Though didn't mind the single boss ones either. Trial was a little janky though but the fights were fun. The big progression guild we were part of up through burning crusade wanted to leave for greener server pastures, a handful of us were kinda done with hard progression and the cuthroat nature of it. Picked up a few more and did 10mans/hardmodes mostly. Hooked up with another guild like us for 25man but we only cleared those hard modes once.. 25man was still a management nightmare...
Played through pandaland, skipped warlords, came back for some of legion and then finally broke the habit.