Various reasons. We've been in a precarious situation for a while, and time deteriorates all things. What we know for sure is that within five hundred years... Cocoon would crash on Gran Pulse and obliterate all life on the planet. There is no escaping that scenario, and thus the best method is to create an arc where humanity survives.

A new world, by that time.

And yes, I believe that's possible. There's always going to be-- glitches, when working with prototypes and experimenting, but eventually, they may have a working method of replicating Core energy.