@Romariel Security level refers to how hard and/or costly is to contain it, it doesn't refers to how dangerous it is. Being a bridge that must be supervised 24/7, with all crossing strictly forbidden, I think it must be costly.
I think is more like they still can experiment the enjoyment of growing and learning new magic, while he is at pretty much the epitome of how strong a magician can become, having almost nothing (or maybe nothing) else he could learn now.
Don't know if I got it right, but I'm pretty sure this is gonna end up in harem, given the hints the author has been giving us from halfway the manga, but who knows?? Also, MIKU BEST GIRL!!!
If we evaluate this the logical way, then there shouldn't be any reason to continue being MC's girlfriend once she has been found, so if she really didn't had any kind of feeling for MC then she could had let him die, or dissapear without saying anything.