The sub design in question has no ballast tanks. It literally "flies" downward and "sinks" to the surface. No navy subs (at least non-classified subs) do this. Not having ballast tanks is a huge design advantage.
Firing missiles from a Sub against a low tech country with no navy to speak of is hardly a combat situation. In naval combat the only thing subs are really afraid of is other subs and aircraft, and the best way to give up your position is to fire a torpedo.
Not having a ballast tank is decidedly NOT a huge design advantage for a military sub, because to stay under the surface you have to keep moving at a certain rate of speed. That is not a desirable trait when your primary goal is to be stealthy.
It IS a great design advantage for non-military subs when your primary goal is to be safe.
The concept is not exactly earth shatteringly new, I've seen concepts of it from navy contractors for decades, the Navy just has no use for one.
For surviving, yes, the only thing can kill a sub is a ship or airplane. How for succesfully completing a mission usually the goal is to remain undetected. That is certainly harder to do.
The sub design in question has no ballast tanks. It literally "flies" downward and "sinks" to the surface. No navy subs (at least non-classified subs) do this. Not having ballast tanks is a huge design advantage.
Firing missiles from a Sub against a low tech country with no navy to speak of is hardly a combat situation. In naval combat the only thing subs are really afraid of is other subs and aircraft, and the best way to give up your position is to fire a torpedo.