Interconnecting wheels, with wheels within wheels; interlocking causes and effects.
I do not believe the plummeting oil prices alone can account for the Soviet collapse; nor can defence spending alone account for it, nor the declining respect for the central government resulting from the Perestroika alone.
Some factors are greater, some smaller; some could in hindsight have been deflected relatively easily, others remain more troublesome — but these factors, and other processes going on at least since 1980-ish, add to a vector sum that historically ends where it does.
From my perspective, building a plausible story is more about tweaking the component vectors to bring the sum where it needs to be, rather than introducing large, sweeping cataclysmic events with little or no foundation in preceding causes.