After doing quite a bit of reading, I have come to the conclusion that the Western Allies in general, and Lend-Lease in specific, completely changed the outcome on the Eastern Front. That's not to say the Germans would have had their Thousand-Year Reich, but that it's extremely unlikely that WWII would have ended with the Red Army in Berlin.
Historians who dismiss the impact of Lend-Lease tend to fall into a couple camps. One is ideological communists who clearly have an emotional affection toward Stalin and a constitutional inability to admit those filthy capitalists ever did anything right. These sorts can be summarily ignored and dismissed. They're not serious people.
There are two more common sorts of wrong historians. One is the sort who believes the outcome of a war can be determined by gross inventories. Whoever has the most factories and people will win. The USSR had the most of these, so QED. You see such historians arguing that Northern victory was inevitable in the American Civil War. Somewhat related are historians who focus mainly on armaments. The USSR had more tanks and rifles, therefore the USSR was guaranteed to win.
Historians who focus primarily on bulk & gross numbers tend to have little to no understanding of the specific impact of individual technologies, which can often be enormous. A good example of this is the Russo-Japanese War. On paper, Russia should have won, having more ships and more men. In practice, the Japanese navy's use of electric fire control systems and radio coordination, while the Russians relied on outdated methods, resulted in them crushing the Russian Navy. Or, you could just look at Russia in WWI. They had the largest army, but didn't have the rifles and ammunition to actually equip the army, resulting in defeat by the Germans.
So, on to WWII.
One of the most important revolutions in 20th century warfare was communication technology. During the Battle of France, the Germans infamously ran circles around the French in their tanks, as every German tank had a transceiver, while French tanks communicated with flags. The same thing happened on the Eastern Front during Barbarossa. German tanks had radios, and German infantry had field telephones. Soviet forces had neither of these and took massive losses. The Russian situation was not resolved until 1943, when the United States had sent enormous quantities of radios, telephones, telephone wire, and vaccuum tubes to the Soviet Union. Zhukov's defense & counterattack at Kursk, which broke the Wehrmacht, deserves all the praise it gets, but it simply would not have been possible without the coordination enabled via American radios and telephones. Additionally, the drive to Berlin as we know it would have not been possible without American communication equipment. The USSR's large-scale operations during this period were completely dependent on modern communication and simply couldn't have happened with flag signals and couriers running handwritten notes.
Much more could be said about American trucks and train cars. Since at least the American Civil War, mechanized, electrified logistics have meant the difference between victory and defeat. If the USSR had been on the hook for its own transportation and communication equipment, its logistics would have been as bad, if not worse, in 1943 as they were in 1941. They also would have had the secondary effect of not being able to manufacture nearly as many tanks, rifles, and planes. Soviet manufacturing output actually dropped over the course of the war, but since America was providing essentially all of the logistical equipment their army needed, they could shift production to weapons, where you want as short a line between factory and front as possible. If we cut the Soviet tank count by a third, or alternatively throw them back on horse-drawn wagons for their transportation, Kursk and its aftermath are completely different stories.
The USSR was eventually able to build airplanes that were at least competitive with the mainstays of the Luftwaffe. However, these depended on American aviation fuel, which the USSR at the time was unable to produce. In an alternative timeline where America stays neutral, there's no way the USSR is going to develop its refining technology and build up capacity from 1941-1943. Without American fuel, the famous Yak series of fighters and the Il-2 Sturmovik, which required 90-100 octane fuel for their 1000-hp engines that Soviet refineries could not produce in any useful volume, simply would not have happened. Nearly every single Soviet offensive from 1943 onward (e.g. Dnieper) would be dramatically less effective without air support.
I could keep going on. The T-34 was a rolling piece of crap before an American design overhaul and manufacturing technology enabled the production of the T-34/85. The Red Army would have been much smaller without American boots, uniforms, and food. The volume of rifles, machine guns, and artillery the USSR produced would have been much less useful without American explosives. On and on.
A side note, head-counters tend to vastly underestimate the importance of manufacturing technology. This isn't just a WWII historian problem, it's a problem in modern businesses as well, which has led to people thinking they'll save money by moving manufacturing to China, then being surprised when the degraded quality of goods ends up costing more than it was worth. American engineers taught Soviet engineers how to properly manufacture armor plating after multiple disasters caused by T-34 armor spalling (too hard) or being penetrated (too soft). Additionally, one of the main things we sent over under Lend-Lease was high-quality manufacturing equipment (lathes and such), which resulted in T-34s lasting for less than half their rated life before falling apart to lasting well beyond it, effectively doubling the number of tanks the Red Army could actually put to field. To put importance of American quality manufacturing in perspective, the Japanese captured M1 Garands, tried to copy them, and ultimately failed because they could not forge & mill the parts at a high enough precision for the Type 4 rifles to work reliably.
One more thing, however, deserves some significant address. Without the UK holding on and the Royal Navy gaining the upper hand in the Battle of the Atlantic, the Wehrmacht's fuel problems and the eventually disastrous drive to Stalingrad would never have even happened. Had the UK surrendered in 1941, Operation Barbarossa would have happened with ample imports of fuel from around the British Empire. There are other secondary effects - Germany spent more on its navy than every ground vehicle combined. So imagine Barbarossa not just without any fuel problems, but more tanks, more assault artillery, more trucks and half-tracks, and so on. Imagine Panthers and Tigers being produced in far greater numbers. This is on top of the Soviets having all the hits to the army's size and effectiveness due to never getting access to American materiel, as discussed earlier. After the brutal winter of '41, the Wehrmacht most likely would have simply pressed on to finally take Moscow and Leningrad.
I don't think Germany would have eventually won, as the UK dropping out was an absolute political non-starter. Hitler would not have gotten his Thousand-Year Reich or his Lebensraum. America's involvement in the war was ultimately because of Japan, not Germany, and the USSR was still going to tie up enough German resources that Normandy, Sicily, etc would have eventually happened, and Germany would have been stuck in a two-front war one way or another. However, without Allied help, I think the USSR's fate would have been much like the Russian Empire's in WW1, being pummeled by the Germans until broken beyond belief.
