It's wrong to say that the 'Russians' would have lost WW2 without allied support, but it's equally wrong to say that the 'Russians' won WW2'.
As it was, Germany came close to cutting off the Soviets from its oil supplies with its offensive operations in the Caucasus. This would most likely have been fatal. Without allied support, such as the Artic convoys, lead lease, and intelligence(remember the British were supplying the Soviets with intelligence derived from Enigma decrypts), The intelligence provided by Britain likely contributed to the Soviet preparations for significant battles, including Kursk in July 1943. At Kursk, the Soviets could anticipate and counter German attacks effectively, partly due to British intelligence of German plans.
Allied naval superiority played a crucial role in securing raw materials. It facilitates the protection of supply routes, sustaining allied war efforts(including Russia) while denying Nazi Germany the same raw materials. For example, coal-based synthetic fuels accounted for approximately 60% of Germany's total oil supply by the war's end. Amazing that. Strategic metals, rubber and chemical precursors, all crucial for aircraft and munitions production, were always in critical supply - all thanks to allied naval superiority.
We have yet to factor in the US industrial and economic might. Would the US even enter WW2 if Churchill was pushed aside and Britain cuts a deal?
With the US and Britain out of the conflict/neutralised, Germany would have redirected its military resources and strategic focus entirely on Russia. Is the result the same? Hmm. Yeah, Nah.
Churchill is a giant, and no praise can be considered too much in proclaiming his role in saving Western civilisation. Don't get me started.