The problem is that by incorporating Afghanistan, the USSR would undermine the legitimacy of its borders.
Part of the post-WW2 settlement (greatly reinforced by the Helsinki Accords of 1975) was that states could not interfere in such a way as to change the borders of other states. That means no annexations.
By breaking this, the USSR basically hands over a giant permission slip to the hawks of the USA, who have far more power to aggressively change borders than the USSR could ever bring to bear.
Most likely the US does not challenge the USSR too directly, however, from the point of annexation on, the Cold War would get very much worse as both the US and the USSR try to annex some place or (more likely) have a proxy annex some place that they think the enemy won't care too much about, only for the world to find itself in a Cuba-style showdown.
Certainly, one of the places where the US does increase pressure on the USSR is over the Baltic states.
The US will also try to discredit the USSR as a patron of 3rd world states by asking people "do you really want to be an SSR?" That effort will probably go rather well. Most of the communist/socialist movements the USSR aligned with were highly nationalist and would be quite unimpressed at Soviet actions in Afghanistan in TTL
Not sure if the US would still support the Mujahideen. If they don't, the Mujahideen are utterly crushed (but not easily) by the Soviet security apparatus. No guerilla force can exist without external support.
We were lucky to make it through the OTL 80s without a nuclear holocaust. TLL, I suspect civilization is doomed.
fasquardon