They seem to be expecting Shadowbringers to significantly boost playercount.

Otherwise, the split on EU makes no sense given that even with peaks of players upon ShB release likely won't surpass what JP/NA data centres have been running on as standard.

That said, maybe they've got some data that supports growth. Such as retention numbers throughout SB. I mean, playerbase growth is what prompted them to release Gold Saucer Mage. So who knows, maybe things won't suck horribly...

Their main issue is they need to be REALLY sure about something like this. As they can't just merge worlds if populations are being too spread out due to housing. While if they remerged a data centre, they'd make a lot of people who transferred and lost their house and furnishings quite unhappy.