Nuclear weapon proliferation seems to have plateaued recently, but I think that this appearance is partly deceptive. The main reasons it has plateaued is that: 1) building and maintaining nuclear weapons is expensive, 2) there are powerful countries that are willing to use military force to stop some other countries from developing nukes, and 3) many countries have reached nuclear latency (the ability to build nuclear weapons very quickly once the political order is given to do it) and are only avoiding actually giving the order to build nukes because they don't see a current important-enough reason to do it.
The prohibitions aren't perfect, of course (and not without their own negative externalities in some cases). But all of those things are much more accessible to people than nuclear weapons, and we've still had successes in banning/reducing them. So maybe there's hope yet.