I had originally planned to rewrite this piece. However, time constraints have worked against me and so here it is.
There's plenty of hand wringing over the US upgrading and replacing its nuclear weapons. The talk is the US will start a new arms race. This is under the assumption the US will be causing others to upgrade and replace their current weapons. This is untrue, but not for the reason the critics think. If the situation was static, the US the country doing the upgrades first, then they would be right. Rather the Chinese and the Russians have /already/ started their replacement of nuclear weapons, delivery systems and carriers for the same.
Russia just laid down its *7TH* new nuclear missile submarine. The Chinese just tested a rail mounted (as in railroad) nuclear missile in a railroad car intended to look like any other. China and Russia are working on new nuclear bombers. Russia has developed an unmanned sub/nuclear torpedo with a dirty bomb. The Russians and Chines have developed new cruise and ballistic nuclear missiles. etc. etc. The list goes on.
Its hard to see the supposition the US is causing the nuclear arms race when we have barely even started the replacement of old systems: the B-21 program has just been selected. The Ohio Replacement Sub is just barely starting tech demos. The Long Range Stand Off weapon and Minuteman replacements are just attempting to get into the budget; nevermind, even done anything other than papers on the subject. This is reactionary, not causative: they started /after/ the other two rival nuclear powers have long since started upgrading and developing their new nuclear capabilities.
It could be argued the missile defense installations, ABMs in Alaska and Aegis Ashore in Europe, could be driving the Russians and Chinese to do their upgrades. At first blush, this seems somewhat reasonable. However, the overwhelming capability of the Russian arsenal, even if 75% failed on launch, would still easily overwhelm the missile defenses in place. The only real defense would be against launches well short of a total war: if a nation wanted to fire off a handful of nuclear missiles, then the defense would work. If it fired off over 100, then we're screwed and there's no way, economically, to stop an all out attack by even old nuclear weapons: 250 nuclear weapons going off over targets all over the US would sink the country, no two ways about it. People would survive, even the country might survive, but we're done as far as being able to wage a war or be a threat to well, anyone other than Papua New Guinea. So, claiming US ABM efforts are driving the nuclear weapons race seem to fall flat as that it only eliminates a 'limited nuclear war,' something most think was a fantasy anyways.
The whole premise of nuclear weapons is as a deterrence and if that deterrence is not credible, then the whole point goes away. The point, from the nuclear powers POV, is to prevent any major attacks on the homeland because the consequences are so great. In some sense, ironically, nuclear weapons may be the biggest peacemaker the last century: the cycle of devastating wars in Europe between rival powers was ended and, indeed, prevented just as it was about to go global from doing so.
This begs the question why "experts" are arguing for NOT replacing the old nuclear capabilities. Its not really because we would cause an arms race. That race is already happening, whether or not we are participating. Nuclear weapons have prevented major wars despite, or potentially even because, of their devastating effect if used. There are only two reasons that come to mind. Either someone is morally opposed to the potential devastation caused by those weapons, understandable, if a bit misguided or the advocate lacks an understanding of how the world has changed in the last 25 years. The former will argue for the the dismantling of the nuclear arsenal no matter what. The latter hold to the position the US is in an unassailable position as the overwhelming superpower, a so-called hyperpower. That last is semi correct, but...and there's always a 'but.'
The 'but' is three fold. The world changes and the US is now faced with nations rising, even converging on the US; the US has a rapidly aging arsenal; and technology has marched on.
China is rapidly closing in on becoming the number one economy in the world. Russia is no longer utterly economically devastated by the break up of the Soviet Union and while crippled somewhat by sanctions and oil prices, has elected to invest heavily in its own arsenal with a far better economy than it had during the dark days, from their POV, of the 1990s.
Secondly, entropy wins all wars. Even with maintaining the current US arsenal, many parts of it are, well, virtually ancient. The B-52s, for example, went into production in *1955* and the the newest of them was built in 1962. Yes, our backbone bomber is minimally 53 years old. Even with spare parts, these aircraft will wear out. The Minuteman III despite its upgrades dates to the 1970s. The Ohio class submarines were introduced in 1981 (older than some congress members). All of these will wear out. Their spare parts may not, in some cases, even exist or even be able to be manufactured at all: the Minuteman missiles use a physically gimballed inertial navigation system for example and lets not talk about the guidance systems... Something the rest of the world moved on from 20 years ago. Or more. Again, entropy really does win all wars.
Related to the fact the equipment is so old, technology has marched on. As many technologies easily surpass what was incorporated in the original weapons and allow for others to develop their own, novel capabilities: technology is not strictly American, especially in the world where we have peer and near peer economies can afford to develop their own. It is not inconceivable Russia or China, especially China, may come up with a method to detect our Ohio class submarines while at sea, removing their deterrence. This enters into the unknown unknowns of human creativity and the longer a problem exists - the US nuclear arsenal and its capabilities and limitations - then the more likely a solution will be found fostered by new technological capabilities.
In the end, it makes little sense to NOT replace what the US has for a nuclear arsenal. The 'destabilization' caused by an arms race has already happened: the Chinese and Russians are rapidly developing and deploying their own new weapons. The Russians and Chinese have become very assertive, pushing back against the international order: Russia occupied and changed the borders of another country in Europe. The Chines are laying claim to virtually the entirety of the South China Sea. These are not the acts of friendly nations. The so-called 'Strategic Holiday' has ended: Fukuyama was emphatically wrong. To neglect a tool, weapons capabilities that have kept the US homeland secure for 70 years is worse than short sighted.