The technology to make small, compact, high output thermonuclear weapons isn't out in the open nor is a lot of the technology used to reduce the CEP of ICBM warheads.
The big thing we're missing for modern weapons is the nature of aerogel between the primary and secondary stages. You'll still get a nice boom if you screw that up.
Basically we know why we want it (optimally igniting the second stage using the energy of the first stage), but we don't know exactly what it's made out of or the process for making it. We know it's gnarly, toxic stuff though.
It requires nation-state level resources to even attempt to build them. Even states dedicated to developing them against the wishes of the international community (i.e. North Korea) have been barely successful.
It once required the smartest people utilizing resources from the wealthiest nations. 80 years later North Korea is making a pretty good effort. In another 80 years do you think it's unreasonable to think a wealthy individual might be able to build something?
In other words, why would building an nuclear weapon not get easier?
> In another 80 years do you think it's unreasonable to think a wealthy individual might be able to build something?
They'd not be able to do it without the weapons grade material. The matter of getting that stuff isn't really a secret, it's just pretty expensive, takes a lot of time, and will be noticed by intelligence agencies. I don't see why any of that would change.
The list of suspects will be short; anybody trying to refine U-235 would need huge quantities of natural uranium and a warehouse full of precision centrifuges. And anybody trying to create Pu-239 would need a nuclear reactor.
