I think shipper fans who have been waiting for them to get together, would be pissed off if they eloped, plus that doesn't seem like a thing Kate would agree to.
I don't want to see Bastle elope. And a part of me thinks they should ONLY REALLY get together when they are wrapping up the show.
I don't thing a post coitus Bastle, or Caskett if you wish, would be as interesting.
Maybe for the movie. After six seasons.