SSGNs have at least 2 major roles:
1) Special Forces insertion: With lots and lots of empty space
in the ex-missile compartment, they can carry special forces and
insert them by mini-sub. Perfect for island fighting.
2) With 154 Tomahawks, they can do anything from taking out
conventional land targets, to anti-shipping, to anti-runway, or
even low-yield Nuclear. They are far more direct uses than SSBNs,
which are there more or less for deterrence.
With the US the de-facto guarantor of world peace nowadays,
anything the Chinese do to the Spratly Islands will definitely
affect the US, since it directly affects two strong allies
(Philippines and Taiwan) and the whole of South East Asia.
The British are going to build 2 new carriers, and the Germans
are currently the world leaders in silent non-nuclear submarines. I
think the Atlantic can take care of itself.
Besides, none of the Russian ships are new, so the US can
probably counter them pretty easily. The Chinese, on the other
hand, have been diligently buying Russian technology and melding it
with stolen US technology, so the US is probably more wary of a
surprise coming from China.
