The absolute worst outcome probably involves Gorby never coming to power or eating a bullet early on. If the Soviets get some hardliner back in power, one who refuses to acknowledge the need for reform and tries to hang onto the SSR's and Warsaw Pact through brutal force, the ensuing low-grade conflict will eventually blow up into civil war between and within the SSR's themselves, and NATO intervention in the former WP countries is a likelihood. Most of the SSR's wanted to stay united as long as it was clear that reforms were being undertaken, but none would have been willing to stay in without reforms on the horizon. The resultant war would have been very, very bloody and might have even gone nuclear with all the weapons lying scattered, and poorly secured, across the country.
In short, Yeltsin was an idiot because he wanted to throw out the entire system, but keeping the whole thing would have had an even worse outcome in the end. Gorbachev had about the right idea; let go of the parts of the Union that wanted out, negotiate a federal structure with the rest, and reform the economy into Sweden writ-large over the ensuing decade. You can't challenge the US and NATO on its own terms, but you can vastly improve the lives of the average Soviet citizen and neutralize Eastern Europe as a buffer.