You were able to switch to Eos at a later point because some other people must have gotten the same Restriction message as you did and joined Dione, which then caused Dione to have equal numbers and allowed people to join Eos again.
I understand your concern. The game is not forcing people to join the realm they dont want to be in and then have them pay 50k. It is a balancing system, it works like a scale.
If the Eos side is too heavy, you have to put weight on the Dione side to balance the scale. This is how the Realm Balance system works.