You're assuming a very limited social network to be constructed within this hypothetical server, if you think you can just tell them all to move somewhere else together. The worst case scenario is that they all want to move to the same server, and then either that server becomes congested overnight, or the community is fragmented after all which you assume is something that won't happen. Another case is that one group whose trial ended moves, the next group wants to follow, but somewhere in between that server became locked and that second group can't go where they wanted to. It's a situation which likely means some of those players won't continue to be subscribed for much longer.
Letting people choose which server to create their new characters on is one of the few natural ways in which this is circumvented.