If it was just Germany vs. USSR there would have been 5 years of fighting in Poland, because neither side would be able to advance much past there.
What is more, much of the early Lend-Lease aid was unusable. British tanks were not what the Red Army needed, and British Army greatcoats (like German greatcoats) were totally unsuitable to the Russian winter.
The Soviets had already gained the upper hand on their own account before Western aid began to reach them in quantity.
One does not need to look further than the second week of 1943. At that point in the war the very first US soldiers set foot in continental Europe (on a distant beach in southern Sicily). At the same time, on the Eastern Front, the Red Army was breaking the Wehrmacht's back to such an extent that the German war machine would never regain its offensive capacity.
Western aid may have been something more than the icing on the cake, but it was not the decisive factor.
Maybe the Soviets were more successful on the battlefield than the Western Allies, but how does that prove that aids sent to the Soviet Union in shape of supplies and hardware were not decisive?
Also, it's not quite true that the Soviets had devastated the Germans in the first two weeks of 1943. The Germans hadn't even capitulated in Stalingrad at that time, although it was obvious they would not win the battle anymore.
The Red Army had already gained the upper hand on the Eastern front at the turn of 1942-1943 before the full weight of Western assistance could be felt.
The route via Iran indeed ensured that the Red Army lacked for nothing. From late 1943 flowed an uninterrupted stream of goods, but by that moment the Soviets were doing not that bad already. Yes, there were also Arctic convoys (with two gaps with no sailings between July and September 1942, and March and November 1943), but the most aid the Russians got flowed via Iran.
The Pacific Route opened in August 1941, but was affected by the start of hostilities between Japan and the US; after December 1941, only Soviet ships could be used, and, as Japan and the USSR observed a strict neutrality towards each other, only non-military goods could be transported.[21] Nevertheless, some 8,244,000 tons of goods went by this route, 50% of the total.[20]
I would dispute that. Please explain what you mean by the full weight being felt. What is your source for that anyway? Moreover, as I already said, the Soviets having 'gained the upper hand' really is a very vague statement. It sounds like the Red Army had a cakewalk after driving the Germans out of Stalingrad. True, the Red Army lost no major battle afterwards, but the effort it took was nevertheless tremendous. The Battle of Kursk, resulting from a German counter-offensive, had almost the same scale as that of Stalingrad. The Battle for Berlin was even bigger. Both were triumphant Soviet victories, but they also could have been devastating defeats that would have shifted the balance once again. As Loosey said: German defeat probably was inevitable, but by no means easy.
The Soviets were starting to win the war in part because they could develop their strategies based on the reliability of Allied supplies. What is interesting here is that we have two opposing statements: Loosey said that the Pacific route to Vladivostok was the more important one, Foro says the same thing for the Persian Corridor. I'd be interested in seeing both of your sources for that. Moreover, just because the Pacific route was interrupted twice does not mean that the total of shipped goods on that route was not bigger than those via the Persian Corridor.
WW2's real turning point was Dec. 7th, 1941. The day the USA entered the war. The day the Germans fell back from Moscow. After that, it was really just a matter of time. The USSR alone? I honestly don't know that they could win.