Well, from old US servers only Prophets and RA still exists. Other guilds are either completely dead or they have lost so many players that they are capable of killing only some easier T4 bosses with big support from outside the guild like RN. Hmm good example how many players left the game is RN, I remember during the 5th anniversary there were two full RN raids in Tarantia waiting for the boss,50 players online, mostly T4 geared people, and one year later they can`t fill their own raids anymore or kill Zodiac/Emperor
There are more people playing on EU side but some of guilds from this list don`t exist anymore (or are very good at hiding) and most of them are spamming recruitment adds in global which means that they lack players. And why would people still play, last real content we got-Coils was almost a year ago and we will probably never going to see next raid tier and if they finish and add Cetriss gear from it would be mostly the same or marginally better than T4, they already said that there won`t be much more vertical progression.