It's almost 2 decades old and replacements are supposed to come in next few years.
Again, you can theoretically slap that on anything that can carry a decently sized cruise or ballistic missile. Anything.
Whatever is decided to be most optimal for the role and fits the weight/size restrictions of the warhead.
100-150 kg range, which is what S-300's are at, is already pretty damn generous compared to artillery shells, while 400-500 kg is the norm for cruise missiles, and that can fit any reasonable warhead - ye olde nuclear Tomahawks had 200kt, and it could easily take a stronger warhead if anyone wanted it (strategic weapons do have heavier warheads that weight less).
Artillery shells themselves, due to size, weight and robustness restrictions generally didn't go beyond 2kt for 6 inch class and 10kt for 8 inch class, with rockets and cruise missiles that have warhead weights around mid to high 3 digit kg, there are plenty enough options.
This is a good example of... peak 50's nuclear miniaturization.
en.wikipedia.org
203mm, 10 kilotons, whole shell, not just warhead being 110kg. With experimental 40kt one cancelled but probably doable. Imagine what kind of stuff could be made with today's tech, especially if you plan to put it on top of a missile, without making it robust enough to fire out of a cannon.
Those ways are not really effective for detecting flying aircraft with nuclear weapons, nevermind missiles, otherwise stealth nuclear bombers would be pointless. They work best for... slow and methodical scanning for stationary facilities. Apparently it's not that great even for something as mobile as trucks, considering how Russia, China, NK and Pakistan still aren't scrapping their truck TEL based strategic missiles.
So the argument is that the base in the plans as they are now is completely fine, but in the future theoretically USA could build a completely different base on top of it with completely different launchers that will contain completely different, bigger missiles that are exactly what Russia is justified to fear now!
(Though let's not mention that USA could equally theoretically slap these hypothetical nuclear hypersonics on a SSGN and more sneakily part it anywhere on Med, Baltic or polar seas for unpredictable trajectory for better effect if it really wanted to go for a decapitation strike, ditto for stealth bombers).
Ok then, why aren't we treating every Russian presence anywhere, if it contains any missiles of ABM class or larger (aka S-300+), as if it was going to be replaced with dozens of nuclear tipped Zircons, Sarmats or Burevestniks sometime soon?
Yeah, because that would make us look cray-cray.
If Russia wants to do that, that means either they have gone cray-cray, or are pretending to in a malicious quest to get real concessions for mere cessation of such pretense, in which case the same should be done to them in retaliation.
It's not a lie if you made up a convenient verbal promise (not a deal, some kind of idiotic good hearted promise made for nothing given in exchange that both sides knew that could not be binding due to nature of democratic governance), and never spoke of it until a reason to be butthurt at the West was needed, but not before.
Meanwhile Russia has broken an actual promise made in writing in Budapest Memorandum.
