I've been thinking about this (timing between the two countries vs just timing in the USSR) and you're 100% right. We might have ended up with a more humane and more internationally cooperative USSR vs a failed one (same as if Gorbachev had succeeded alone) but we might also have had a second Carter term as he might have gotten cooperation with the USSR on the Iran hostages issue - and without that issue to hammer voters with, Reagan (and his undoing of LBJ's Great Society) might not have happened. IMO that undoing has been a mostly unmitigated disaster for the US. That said, I think the Civil Rights bills that happened during LBJ were a precipitating factor for our current "great sort", so the Republicans might have lost their minds no matter what.