Hey, man. I replied to you above sarcastically because, frankly, i thought your answer was kind of dismissive.
You're posting here a lot and it's obvious you are really passionate about the old school servers and I truly understand where you're coming from. I'll walk with you through memory lane, because I fucking loved how servers felt like communities way back when.
We had UO, where soloing was possible, and a lot of people did it. It was super dangerous and highly inefficient, mostly due to monster hp and gank squads /thieves. Doing things in groups increased efficiency and safety. As a thief, doing things in a group increased our chances of getting away with loot. Good times.
This kind of held true in Everquest until later levels. Soloing was possible (for most classes) but grouping made securing campus significantly easier and more efficient.
You mention WoW, and I myself hated on the LFG tool as the final nail in the feeling-of-community coffin.
But some other aspects of these games back then? Lower server populations. Smaller game worlds. Longer leveling times/dwell times in individual zones. Decent/unique drops in lower level content. Gated end game content.
People knew other people largely because they had to and because there weren't many people, and that definitely also added a lot to the games that most games no longer have.
But as those server populations burned through content (and low level drops were farmed out or no longer needed or nerfed to pointlessness) who was left in the low level zones? Alts and new people, and the numbers were never, ever in line with how the zones were balanced. And then once they made it through the zones and proceeded to start the dungeon grind that everyone else completed early into release/expansion/what have you, there was the wait. The wait for a group. Any group. Hopefully you could convince a guild you were worth the time it would take to get you keyed/attuned/whatever, cause the wait for that was often insane as well. Waiting and the feeling of emptiness is what kills mmos, as it kills momentum and gives you plenty of time to wonder "why am I paying money to stand around in a game?" The top-heavy population of all of these servers crushed any reasonable chance of meaningful grouping before end game without someone else taking time out to help someone out. I distinctly remember the people trying to find dungeon groups when everyone was grinding out T5 in Burning Crusade. I wonder if they ever found a tank for slab...
Most devs realized this and either streamlined the leveling process, incentivized repeating content, added cross-server capabilities or some combination of these.
Further, every time devs experimented with locking class features behind group content (spells in AQ20, as an example) the decision was rescinded the next content patch, never to return.
Finally, according to Blizz several years ago, players consume new content at paces they can no longer contend with.
Right now in GW2, HoT zone populations are the highest they will ever be. Megaserver is an awesome thing, but it will not provide an endless stream of players forever.
Eventually, someone will be trying to unlock their class features and won't be able to do so, and it'll probably happen to the off-peak players before Christmas. The wait will be too long, momentum will be lost and other, more interesting things will crop up.