I don't know how to answer the part about how healthy Everquest is.
Pardon my paradox, but Everquest is eternal, at least until it dies.
There are far more high level characters than anything. Though there are always people trying it for the first time, making alts, buying extra accounts, returning from other games or coming back to try out a new expansion.
Plat is pretty out of control in general, but prices vary by server. Stupid amounts of plat are only needed to twink though. With the new armor quests and tasks from The Serpent's Spine expansion you can level up and stay moderately well geared. There are also many more ways to earn plat than the old standby of Spiderling Silks these days. Lots of mobs in TSS zones drop pricey vendor loot and even decent gear upgrades sometimes.
The graphics get shinier and fancier with every expansion, in general. They have updated many character models over the years. Orc, werewolf, gargoyle, rat, bat, spider, lots more. They have revamped some older zones to more modern graphics. The newer zones and the revamped zones give some people noticeably more lag.