Had the USSR capitulated to the Germans, there was a real risk that the UK would follow as the German war machine could refocus its efforts. India would likely have fallen soon after to the Japanese. At that point, the German production base, which was already heavily geared up, would have access to all the resources it could possibly need, and the US would have had serious trouble defeating them. It would be a race to the A-bomb and who could produce them the fastest most likely, although it’s questionable how effective the weapon would be with a consolidated Luftwaffe without a continental power keeping them busy.
Without the British, intelligence efforts against Germany would have significant issues. It’s possible that the USSR would capitulate due to this.
Without US lend-lease, the USSR would have capitulated as well, and with only the British standing against Germany hope would have been lost.
The German war machine was extremely powerful. It could not keep going forever of course, and in due time they would have failed. But had any major power not been in the war, Germany could likely have consolidated enough power to avoid successful invasions from overseas powers.
