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"
If the Soviets are able to resist the urge to intervene fully, then they would save themselves from that embarrasment. The problem is that they will need some of the their political and military leadership that the sending "advisors" into a place like Afghanistan seldom stays low key.. Plus all the particular dynamics of that place.
The Soviets will still have a drain on their economy from the massive arms race with the West. There was no way the Soviets could keep up with the defense spending the they had engaged in the 70's and early 80's. Regan was rebuilding the U.S military with huge projects. Strategic nuclear weapons systems are very expensive to build, maintain, and upgrade.
Soviets internal political and economic corruption was beyond belief. In short the Soviet economic system had failed to address the internal domestic problems.
Anyways what could happen?
1. The great war for Muslims will be focused on Israel probably.
2. Iran will be looking to fund up Hezbollah.
3. The typical operations against the West.
4. With the Fall of the Soviet Union the Radical indoctrination of the Southern republics will probably continue.
5. Muslim fighters have their choice to fight in Checnya or in the Balkans as the massive communist police state is no longer there to keep order.
6. It could be very possible that Pakistan losses out several billion dollars worth of U.S and Saudi economic/military aid in which they use to get there nuclear weapons program going in the 80's. The U.S wanted a powerful Pakistan if the Soviets overran Afghanistan. Without all money from U.S and the Saudi's Pakistan would have stuggled to get the bomb and keep pace with India..