> Don't you think that it's much more plausible a scenario where the regime needed a lot of cheap work…

It’s pretty clear that in at least a couple periods of Stalinist history, extreme paranoia led him to imprison or otherwise dispose of enemies in great numbers, e.g. 1937 and again with the Doctor’s Plot toward the end of his life. The late 30’s intensification is, after all, know as the Great Purge and not the great worker shortage.

Anne Applebaum’s scholarly account of the gulag is a great read on the subject. I think she would probably agree that the it wasn’t an either/or issue. Labour demands for industrialization were a driver as were Stalin’s own delusions.