> This kind of dictatorship and purges of competent people certainly lessened the capabilities of Soviet Union, lead to the unexpected result of the Winter War, and greatly increased the losses of USSR in the coming great war.

It's quite a feat to lessen your capabilities, and still be able to repel an invasion by all of Europe (sans England, as well as smaller countries like Switzerland, Ireland etc.)

The other view is Stalin had loyal cadre around him during the war, and did not have lieutenant colonels with links through the high command making bomb attempts against him.