Although my name isn't Vyvyanne, I will throw in my 2 cents.
Strictly looking at it from a technical point of view (but also from an organizational point of view) it makes most sense to KEEP the worlds with the largest communities and player base, and CLOSE the worlds that have the fewest. It wouldn't make sense for instance to close Brandywhine and keep Riddermark. The way they are doing the migrations is they will do it 2 worlds at a time as to not overload the system, and at the same time they will monitor to see where everyone is going. If there is some sort of mass organizing on the first 2 servers, and 80% moves to say.... Landroval, they may decide to close Landroval after the first 4 worlds because it is getting full. (Of course this is highly speculative, and probably won't happen).
Keep in mind that Turbine has never done this before. They have handled individual transfers MANUALLY until some time in 2014, when I transfered my toon from Windfola to Riddermark back in the day, it took 4 days because it was a manual request. They have since automated it with transfer tokens, but the amount of transfers was minimal. Maybe 50, 100 a month at most. Now we are looking at a situation where thousands of players will migrate at once, on a largely untested new system. This is why they will open up transfers between the 5 keeper-worlds first so they can really see how it works and performs when players jump from server to server. They need to make sure it works. And although I am sure they have an internal lab (not bullroarer but internally) that they can use to test stuff, you can't really know how it will all go down until you go live.
Over the coming weeks, it will start to get a little clearer which servers are starting to fill up.