Modernization is not Sustainment

While you'll often see "modernization" and "sustainment" thrown around in the same context, it's actually incredibly important to nuclear deterrence and stability that people understand they are not the same. Take, for instance, this quote from a GAO article:

The Department of Defense (DOD) plans to replace or modernize existing triad platforms including submarines, intercontinental ballistic missiles, and bomber aircraft

Do you see the distinction? No? How about below?

The Department of Energy (DOE) plans to modernize its nuclear infrastructure to life extend and produce warheads and bombs.

The important word here is "produce"

Think about your car (or, if you like, the bike, bus, or train you take to work). Eventually, given enough time and miles, that machine will have a part that fails. You have two choices!

1. Buy the exact same part and replace it ("sustaining" the vehicle as-is)
2. Buy a newer model of the same vehicle (a "modern" version of what you have)

For the first option, you can only really do this for so long. Your manufacturer probably only made so many parts before stacking them in a warehouse and retooling the factory for a different thing. If you've got billions of dollars (like the US government) you can probably force that factory to reopen and produce more, but this may not make sense if the technology involved is obsolete.

In the second case, consider the transition from smaller cars to SUVs in the US over the last 30 years. Your old car was a nice, mid-sized vehicle that worked well and had decent gas mileage. Buying a newer model, however, is a huge change that you might not want: it's a much bigger, heavier, faster vehicle! It fundamentally changes the way you can use your car!

Getting back on topic...

Example 1 is what the US ICBM fleet has done in recent history. Sometimes referred to as "refurbishment", quite often these programs are basically rebuilding the whole missile from scratch anew, with the exact same specifications. Which is to say, the fundamental strategic implications can't really change because your capabilities haven't changed.

However, for reasons both valid and potentially concerning, the Air Force isn't exactly excited to continue this forever. For example, it's true that old electronic components and chips are no longer in production, and at some point a redesign will be needed. (Note, however, that this doesn't have to necessitate upgraded capabilities). That's where the invalid part comes in: just because sustainment currently doesn't make sense (by requiring we use obsolete tech no longer available), doesn't mean we should jump to modernization instead.

So what is option 2, modernization, really about? Money. No, seriously: Northrop arguably absorbed Orbital ATK specifically for the reason of becoming the sole provider for GBSD and MDA contracts. (The conclusion of experts like Matthew Korda seem to accord with this conclusion as well). The fundamental problem is that the companies bidding on these contracts don't really care about the fundamental danger that improved capabilities might create.

Modernization doesn't care about freezing capabilities where they are. While it's entirely possible for them to stay the same, engineers conveniently overengineer systems beyond the desired spec, and modernization opens up the door to building more precise, fast, and potentially devastating weapons systems. (The book "Inventing Accuracy" has a wonderful case study on this phenomenon and how it "accidentally" changed the entire posture of nuclear policy). This is all to say, when the US plans to spend hundreds of billions of dollars on a modernization program, other countries take notice.

tl;dr

We fundamentally can choose between keeping our existing nuclear weapons (sustainment), or building bigger and better ones (modernization). In fact, Modernization kinda arguably locks us into likely proliferation for several decades, while there's nothing wrong with sustaining existing capabilities to see what arms control opportunities come along!