If somehow the Soviets would let hem become succesfull he would become a Gorbachev: liberalization would lead to the abolition of communism.
The Czech people, given any amount of freedom, would never support staying communist if they had any real choice.
Dubcek would have to make a choice soon: either tighten rule and close the borders or abandoning communism altogether.
Considering that popular reformists elsewhere in the Soviet bloc (like Gomułka in Poland or Ceaușescu in Poland) became hated tyrants with the passage of time, I have no doubt Dubcek would face the same. Though I'd expect him to be closer to Gomułka in how he ruled (that is, not a tyrannical nutjob).
And I don't think liberalization itself would abolish communism, but only very kinds of specific liberalization - ending the Party's monopoly on power, for example.
Alexander Dubcek was the Czechoslovakian Party Chairman in 1968. He sought to slightly democratize and liberalize in order to regain popular support for Communism. He never sought to leave the Eastern Bloc, merely reform it into something more palatable and less dystopian. IOTL his reforms set off the Prague Spring which was met with Soviet invasion. What if/How can he be successful in reforming Czechoslovakian politics and could these reforms spread throughout the Soviet bloc? Could this be the first step in a democratized socialism rather than the authoritarian bureaucracy IOTL?
Interestingly, I found out that Suslov (famous for being a hardliner, overseeing the pacification of Lithuania after WW2 and for being the Communist version of the Pope between Stalin's death and his own death in the 80s) was an advocate of giving Dubcek a free hand. A surprisingly liberal position from the man. So the Soviet intervention not happening is entirely possible.
I think the trajectory of Dubcek's reforms really depends on 2 things:
1) Can he re-energize the Czechoslovak economy (which had been growing disappointingly slowly in the mid 60s)? If so, then Czechoslovakia will be seen as a useful case study for other Stalinist regimes trying to liberalize and re-energize their economies during the 70s. Especially among the E. European satellites, who were more alike to each-other economically than any other Stalinist regimes. If not, then he gets dismissed as another failed experimenter who might have some useful ideas, but is generally not someone to be emulated, just like the other attempted reformers.
2) Does the complete lack of censorship in Czechoslovakia lead to good things happening? Is it a valuable safety valve that strengthens the "socialist" society, even as it lets those who feel stifled create art to express their feelings in a thriving sub-culture? Or does it lead to humiliating mass-protests and rioting that push the regime to clamp down again?
fasquardon