We have little to no idea if this is indeed inevitable. The same kind of arguments could have been made, and were in fact made, about democracy and capitalism before the American and French revolutions. And some attempts at democratic revolutions have indeed fallen into authoritarian rule - Cromwell's being one of the most well known.

Socialism is nothing more than extending democracy beyond the state to the workplace. It is no more collectivist than democracy is in any other economic system. And like any other form of democracy, it is naturally opposed to authoritarian rule, not conducive to it.

As such, the problem with socialism is not at all that it's easy for it to fall into dictatorship. The problem is that it is hard to convince the rich to allow it to form without aggression, since it necessitates them losing much of their power. The same problem that democracy faced: kings rarely step down, and bloody revolutions are typically worse than the status quo (and you can never be sure what will happen after one).