What will happen to the series? I'd say one of four choices:
1. It will be a huge hit and sell millions of copies like SH3.
2. It will rapidly reach crush depth soon after release and disappear.
3. It will do just well enough to limp along -- a sort of steady state.
4. It will be somewhere in between the first three choices, or an amalgam of two or more. Or not.
But no matter what happens, whether the game succeeds or tanks or whatever, SOME company somewhere will eventually make another submarine simulation. Why? Because they're not stupid. They will look back on SH3 and say, "Hey, those guys made a killing off that game. Maybe we can to, if we do it right."
Unlike some simmers I am not at all pessimistic that if we "don't support UBI" we will never see another sub sim again. There's money to be made in them thar hills, or ocean depths, if you will. Some company, somewhere, will go after it.
So chillax, people.