Half the population that disappeared were all bots.

If you want proof of this, have a look at all the FC's ranked below 40 with spam names like 'asdodadf'.

They are all level 25 bots with rank 6 FC's waiting to buy up all the housing on 6.1 release.
https://na.finalfantasyxiv.com/lodes...ing/fc/weekly/

Watch how congested it becomes in 6.1 when these thousands of bots log back in to attempt to purchase up a ton of land to sell on the blackmarket.