Yes, I am aware that the policy was almost inevitable as a means for the USSR to survive. But let’s suppose whoever succeeds Lenin is extremely dogmatic and remains committed to a worldwide revolution. They refuse to establish diplomatic relations with bourgeoise regimes and continue to support prospective revolutions abroad. How long until they fall to an invasion/internal crisis? What replaces the Soviet Union after that?
I will spare you the long quotes, but basically the concept of socialism in one country in no way contradicts the end goal of international socialist revolution. Socialism in one country merely assumes that socialism can be constructed (i.e. that the means of production can be transfered from private hands to the hands of the state and agricultural cooperatives) in a single country alone, for the time beeing. It was a reaction to the failure of the various European and Asian revolutions in the aftermath of WW1.
The counterargument against socialism in one country was never that the proletarian revolution ought to be international – all the factions in the party agreed on that (allthough Trotzky later made this rather hillarious argument in "The Revolution Betrayed", he didn't do so at the time). The counterargument was rather that the construction ot socialism in one country alone was impossible. The Rights claimed that, while accepting socialism in one country in principle, Russia was just too backward to implement the concept. They advocated for a program they called "socialism at a snails pace" – basically, the NEP was to be continued indefinetly, untill better times were to come. The Trotzkysts on the other hand claimed that socialism in one country was flawed in principle – that any individual socialist state would innevitably descent into bureaucratic degeneracy in the face of capitalist encirclement. Trotzky's theory of "permanent revolution" initially har nothing to do with the international socialist revolution. During the 1920s, he concerned himself with the relationship between the proletariat and the peasantry, and wheater any alliance with bourgeois forces was acceptable if the communist party did not have the leading role (a question which he answered negatively – he came to advocate the opposite position during the 1930s). Only later did "permanent revolution" get a meaning in regards to international affair – and a rather vague one at that. The truth is, Trotzky never publicly proposed an actual plan of action – the Rights at least had a plan with which they confronted socialism in one country (the NEP), Trotzky had no such thing. He opposed the NEP while at the same time claiming that any diversion from the NEP would innevitably result in bureaucratic degeneracy. The only thing he really had to say on the matter was that "revolution export" (i.e. the installation of communist governments in other countries by military force, against the will of the local proletariat) was absolutely legitimate and good (a thesis Lenin vigorously opposed). Later on, Trotzky covertly came to support the NEP aswell.
On the question of how long the USSR could last in such a scenario – not long. Without the crash industrialization, the Union gets steamrolled by the Wehrmacht.