I would argue that it is P99's fault, and that their inability to deal with this problem is the reason the server has 700ish players at peak instead of 3000. So many people have left because they realized the ceiling is artificially low due to the fact that there's no endgame unless you join one or two particular guilds -- very exclusive guilds that demand unreasonable things of their players because that's how you "win" at Everquest.
The problem is that P99 tries to emulate everything about Everquest except one thing: there's only one server. Instead of 20+ servers on which the unhealthily obsessed hardcores can spread out, there's just one server, and their playstyle completely destroys the endgame for everyone else. Everquest doesn't have enough content for more than like one and a half raid guilds, but you could always move to a different server where the endgame hadn't yet been completely monopolized, or where there was room for you in the top guild.
This "feature" is completely missing in P99, and the result has been disastrous. It's the #1 problem of this server, the #1 reason for people quitting, and the cause of endless amounts of grief and conflict. GMs have quit over the constant abuse that results from this, entire guilds have collapsed, and I'd estimate that about 75% of the players who have reached the higher levels on P99 have consequently quit the server after discovering that the best they could hope for was to farm Sebilis and level alts.
Failure to compensate for this is the reason P99 has only ever been merely good instead of groundbreaking and amazing. It's such a strait-jacket on the server, and look at the result: people are
starving for an alternative to the point of giving a criminally bad server like The Sleeper the chance that it never deserved. Every time a new server pops up, hundreds of people flee P99 with hopes of the new thing being a cure to exactly one problem: the clogged and monopolized endgame. It's only because no alternative of reasonable quality has ever presented itself that P99 still sees numbers in the high hundreds.