Of course the bubble is unstable, because you would need something to stablize it--duh. Even in Trek, warp drive is only possible with a stable warp field.
But even if Alcubierre's theory doesn't pan out, it hasn't been the first (nor will it be the last) theory about warp drive to fail.
Hamsters in a spinning wheel hooked up to a veterium cortenide-laced coil. I keep telling folks, that's
the key to warp drive...