Would the USSR still be around if General Zia-ul-Haq hadn't come to power?

[ag]

It had about as much of an impact as the alignment of Venus on May 7, 1089 - pretty much none. I wouldn't seriously consider it among the top 2000 things that brought the Soviet Union down.

USSR was not brought down by the Afghan war, period (though the war would, probably, make it somewhat higher among the causes of the Soviet breakdown - still, far from the top 10).

[ag]

Had the Soviet Union survived in the form Gorbachev had took it near the end, it'd be way better off than most of the ex-republics are now. Russia was certainly more democratic right near the end of the Soviet Union than it is now, and some of the Central Asian republics are far worse off than the Soviet Union was at any point since Stalin (Uzbekistan and Turkmenistan being quite obvious)

Freedom House scores agree with me here too.

You know the old Russian saying: "If the grandma had balls, she would have been the grandpa". There was a major reason why Soviet Union did not survive in the form Gorbachev had taken it: because he had taken it to disintegration.  In order to survive, any Soviet government had to be willing to kill people.  The moment Gorbachev expressed doubts about killing, it all collapsed.