I don't agree at all. Yes, the Soviets were prepared to struggle forever, but imagine if Hitler never faced a two-front war? Or imagine that Japan, unengaged with the USA, was able to send 1 million troops into Russian the Russian Far-East, and Stalin faced a two-front war?
If "by itself" the author actually includes American technology gifts and contribution of 1/3 of the Soviet GNP... sure, maybe.
The spy's fate? Years later he wound up in a purge. When someone reminded Stalin of the key role the spy had played, Stalin said "Gratitude is a disease for the dogs" and had him killed on schedule.
If not for Midway, I think England would have been knocked out of the war because they would retreat from India and concentrate on the Western front. We would eventually stabilize the Pacific, but I think Germany and Russia would have divided up Europe after slugging it out for awhile. You would see Hitler, Stalin, Japan and the US in an awfully precarious position.I don't know what would happen because German technology was quickly outpacing ours. I don't think the ending was looking well.
To this day, the Naval War College cannot duplicate a victory at Midway