It would probably have opened up sooner or later, one way or another. The Soviet Union was collapsing and could no longer afford big armies of occupation. The Eastern nations were boiling with popular demonstrations and desire for change. Refugees were fleeing the East and decimating the workforce. In Germany the marchers were no longer chanting "let us out" but "we're staying here."
Mr Putin was in Berlin at the time as a KGB officer and has expressed regret that the crowds were not crushed by force. He has subsequently stamped out popular resistance in Russia.
But nobody on the spot would take responsibility to use machine guns and tanks. They might not have been lynched but they could well have been court-martialled if change continued.
Once the breach had occurred, the state forces lost confidence, and the citizens gained it. In some of the old photos you will see border guards looking stunned and dazed as they see their world crumbling around them. Eventually they opened gates themselves. As public servants they had a duty to prevent people being injured in the crush.