I'll take your point on that Ender but I think the issue I guess comes down to scale...
Those cruisers are big but nowhere near as big as a Planet.
But If we had a better scale reference to them being destroyed It might make it a bit more plausible.
They can shoot at Endor the Gas giant....
Or threaten to shoot this one...
Add another planet or sattelite in to the equation in the surrounding space similar to our own dead moon perhaps is another alternative