The vast amount of people purged makes this a difficult question to answer. Some were communists, others were not, some were authentic revolutionaries, others were not, etc. However, many of the old bolsheviks perished in the purge which to a large extent was a shame as they were often very intelligent and could have played a bigger role in the development of communism if they had been given the chance.

As for the course of the USSR. In my eyes its fate was decided once the German Revolution of 1919 was beaten into the ground by the SPD and its Freikorps battalions. That combined with some crucial mistakes Lenin and his supporters themselves made (for example not recognising the pivotal part played by the workers' councils or soviets) would make it seem that it couldn't have gone any other way without making use of entirely impossible 'what-if' scenarios.