The US is designing a new ICBM because the businesses that will build it have steered the consideration of the question.

That leaves the question of what the US ought to do. I can't rigorously prove this, but I think that the existing ICBMs are in effect a doomday machine, because a Russian attack intended to destroy them would require so many nuclear weapons as to cause a nuclear winter. Does anyone know whether that is true?

If that is true, there is no need to try to "improve" the ICBMs.