Do you really think that 18 million people were sent to gulags for being antagonists of Stalin? Don't you think that it's much more plausible a scenario where the regime needed a lot of cheap work and because of that kept sending people to gulags for lower crimes like petty theft or even customary crimes like not holding a job?

The educated middle/high class in Moscow was probably quite critic of Stalin's policies and nothing would happen to them as long as they were not loud enough with their criticism.