No one who understands nuclear weapons wants to use them. There is a reason Mutual Assured Destruction is named that, they are not viable to use against a target that can respond in kind! China has ~260 strategic warheads, which may or may not be enough to effectively destroy the US (depends on your definition).
Current US ICBMs (about 500 are left) are armed with a single warhead of 300kt to perhaps 475kt.
Current US SLBMs (288) are armed with up to 8 warheads of 90kt to 475kt (maximum of 1,152 warheads total, for an average of 4 warheads per missile).
The B-52s can be armed with up to 528 ALCMs with a variable warhead yield of 5-150kt.
The only weapons of greater than 1Mt are the B83 gravity bombs, only really deliverable by the B-2.
The only other nuclear weapons in the US are the various strategic and tactical B61 gravity bombs, with various yields of 0.3kt to 400kt.
So, the US has already moved away from large yield warheads. One of the main reasons is nuclear weapons do not scale as well as you would think, and modern missiles are far more accurate than older ones. A single 1Mt warhead has less about 1/3 the destructive potential of 10 100kt warheads, while a single 5Mt warhead is only 15% as destructive as 50 100kt warheads.
Nuclear warheads will only become obsolete if we invent something even more destructive, which I hope we do not, or we figure out how to actually rid the world of all of them, which isnt going to happen.
Basically if we ever get orbital mining figured out nukes would get phased out as KEW would be cheaper/easier and cleaner to utilize.