That’s not going to happen, lol. I could say with 100% certainty that even a full month of transfer availability isn’t going to cause these servers to hit full capacity. What it will do, however, is actually get some people to take the transfers instead of being afraid of making that jump with such a short window.
Also there is no “filling up” of the realms. Go look at Faerlina and Herod etc. that have population figures of 80k+. Pretty sure Faerlina is over 100k. You could still have the transfers going, there is no hard ceiling, it would just cause the current low-medium pops to start having queues as well, but those queues would be pretty tame most likely and the end result is the queues on the megaservers get smaller.
It would be better to have every realm be as even as possible. If they all manage to hit full status, it means the queues overall are going to be massively reduced to playable levels for just about everyone.