There are too many ways for the Russians to degrade American "Launch under attack" capability from sabotage of C3I capabilities, to sudden attack by suppresed ballistic trajectory from submarines and exploiting that Biden-in-the-loop vulnerability. That's why official US position, declared in NDP is simple - Launch Under Attack isn't reliable.
The thing is that the US government supposed to protect American citizens, not to kill Russian and Chinese civilians, committing a sort of "murder-suicide". That's why the first strike must be counter-force and avoiding civilian population. And the Credible First Strike Capability highly depends not only on capability of destroying some Russian and/or Chinese cities but on the capability to survive their weakened retaliation strike. And right now the USA don't have capability not only to destroy Russian/Chinese submarines in sea, mobile ballistic missiles in forests and their strategic and long-range bombers in the air (and even some silo-based missiles will survive), but also the USA don't have reliable emercom (FEMA is a joke, you know), no significant ABD capabilities, and no National reserves necessary for the further recuperation. Which means that even weakened Russian/Chinese retaliation strike will cause unnecessary death of millions American citizens and collapse of American economy. Back in 1962 there was a good chance to fight and win a nuclear war and even to keep the leading position in the post-war world. Right now we don't have that chance. Even in the most "optimistic" scenarios, with unrealistically degraded Russian retaliation countervalue strike (about few hundreds of warheads) the USA will finish its existence as an industrial power in the highly competitive post-war world and will become a poor agricultural appendix of Brazil, India or Europe.