At this point, I imagine it's a "you first" issue. Trust is pretty darn scarce these days, how can you be sure the other side won't just annihilate you the moment they know you can't retaliate? That or be sure that not even one nuke is active or to take a page from guns, "80%" or nearly-active?
Even if we do manage full denuclearization, it'd be pretty darn hard to keep it that way seeing as how we can't even stop North Korea from joining the club.
Who said anything about trust? It would certainly proceed in a lockstep process, with strict accountability and transparency, where the parties agree on a schedule to gradually dismantle all weapons and production infrastructure and supply detailed information at various checkpoints. You can't build a nuclear arsenal overnight.
NK joined an exclusive club and now Kim is rewarded with a visit from Trump. In this version of the world, possessing nuclear weapons is a source of legitimacy for any country. In a non-nuclear world, actions to obtain them will prompt imediate preemptive conventional attacks. Russia, US and EU could agree to maintain minimal deterrence capability against rouge actors like NK, but insufficient for global war.
> In a non-nuclear world, actions to obtain them will prompt imediate preemptive conventional attacks
There are plenty of ways to sufficiently deter a preemptive conventional invasion and North Korea is a perfect example of it. Their conventional artillery poses a significant threat to Seoul, which by itself put a huge damper on any talks about conventional warfare to topple the regime or attack their nuclear facilities.
> You can't build a nuclear arsenal overnight.
True but it's extremely hard to verify that the other party isn't hiding a 100-200 warheads in some remote barely documented base and hid the extra production in lost/modified paperwork. Larger countries with larger militaries just make the number of possible hiding spaces that much higher.
Even if you do establish a procedure, how do you guarantee that the reported number of weapons prior to dearmament is accurate (and thus matches the number of dismantled weapons), and that the other side didn't keep a stockpile in an off-the-books location? That brings us back to trust -- that the other side is legitimately engaging with the process and that the totality of weapons is known, rather than merely reducing their numbers (which would be no bad thing, but significantly different to total disarmament, particularly if only one side goes through with it)?
> the other side didn't keep a stockpile in an off-the-books location?
This seems an implausible strategy on the long run, to organize such a conspiracy you would need thousands of people in the know, from political leaders to technicians. The value of this intelligence would be so colossal that it's unfathomable nobody would sell it for profit or release it for conscience reasons. See the Israel story and Mordechai Vanunu.
Then again, if you manage to have a stockpile of nuclear pits (actual warheads require continuous maintenance, Tritium replenishment etc.) that only 15 people know about, in an non-deployable configuration and hide them so deep that the ultra-sensitive nuclear detectors mandated by the treaties can't sense them, is that a major threat? Their existence might simply fade out of institutional memory.
Even if we do manage full denuclearization, it'd be pretty darn hard to keep it that way seeing as how we can't even stop North Korea from joining the club.