A treaty between the two Koreas has more of a chance to succeed because (it is now accepted by all sides) that the regime in Pyongyang is capable of delivering a nuclear payload into the heart of the United States.
So this is a failure on two fronts: 1] preventing North Korea from developing nuclear weapons and 2] denying Pyongyang the ballistic missile capacity to deliver the nuclear warheads.
So in 1998, the US reneged on its side of the agreement to normalise relations with North Korea in exchange for halting its nuclear programme. The latter then walked away from the agreement and accelerated its nuclear programme.
The two Koreas also have a far greater chance of sorting out their differences and coming to a binding agreement if Washington remains an observer, rather than a participant, in the whole process. It is clear to countries around the world that any international agreement Washington enters into is likely to be violated, reneged upon, breached or repudiated. Russia has similar grievances about the promises made by Washington not to expand NATO into the Warsaw Pact countries, which, were all violated. The US will also repudiate the agreement with Iran to halt its nuclear programme, in due course. American democracy is simply too dysfunctional and unstable to be considered a reliable actor in international agreements or a sponsor of political mediation, i.e. the (non existent) 'two state' peace process in the Middle East.
More importantly, China and Russia have both welcomed the developments. I am sure the two Koreas can reliably engage Moscow and Beijing as co-sponsors of their negotiations and agreements. Later on, the latter can be conditional to the removal of the US military footprint from South Korea, which, is acting as nothing more than a naval garrison for Washington's imperial efforts to contain China, in the South China Sea and the Pacific.