How much exactly would be left of the 25T when the shuttle got back? Probably not a lot. Then there's also the trivial problem of controlled, sustainable, energy positive fusion not being possible yet. I believe if someone were to come up with a process to for fusion, that there is enough H3 in the oceans to keep us going for a while. I'm not certain how much, but I have to imagine that would be more economical than transporting a leaky cargo of hydrogen back from the moon.