Let's assume Taraki preempts Amin and has him killed rather then vice versa. Limited Soviet presence exists (airpower, airborne troops protecting air bases, advisors) but no direct intervention.
Then what? No great war for muslims, no foreign volunteers, no sense of defeating one superpower and wish to take on the other one, no drain on soviet economy.
SU collapses, but maybe a bit later (say mid-90s).
Different "We Didn't Start The Fire Lyrics"