It's one of their main competitive advantages. If a drone launch starts costing millions of dollars, the benefit it has over a traditional aircraft fly-over starts to diminish.

I don't see traditional aircraft taking off from a submerged submarine any time soon. That would seem to be an alternative advantage to offset the cost.

actually, the Japanese already invented that in WWII - http://www.nbcnews.com/science/sunken-japanese-wwii-submarin...

Not true. You forgot the submerged part.