1. For those unable to realise the degree of change in historical events that would derive from this hypothesis:
The improbability of the hypothesis is irrelevant. Unexpected events occur. This hypothesis may exist in recognition of that. (E.g.; Churchill dies on or about the day France surrenders: would Britain's new P.M. and Cabinet stay in a war that was apt to be long, costly, and perhaps unable to be won?) It is only relevant to gauge the change the hypothesis would create & determine the likely results of that change. This has nothing to do with one's whether one believes the result of WWII was good or bad. It is simple analysis.
2. For those who think this hypothesis gives the S.U. an advantage because there would be no gain to other nations to join the Nazi attack.
If so, why did so many other nations or their citizens willingly participate WHILE the Nazis & Italy were at war with Britain? If they would join the Axis despite the potential handicap of possible eventual repercussion on them, then it logically follows they would be MORE apt to join if the handicap was removed, as it would be in this hypothesis. Many governments or individual citizens might want to help eliminate the "foul baboonery of ... Bolshevism (Churchill, 1919 speech to Parliament)" before it took over in their country. In particular, Vichy France might have such interest: Communism had strong support in France. This might be greatly exacerbated if Britain, while on the cusp of making peace with the Axis, still sent the R.N. against the French fleet at Oran. That might literally drive the Vichy into the Axis. (Historically, this nearly happened. Vichy bombed Gibraltar in Sept. 1940 after the British tried to seize Dakar.) Historically, most anti-fascist partisans were communists/communist sympathisers. However, the general political stance in most of the occupied European nations was conservative to authoritarian.
3. For those who argue it would be costly & difficult to keep these armies standing without being used.
Why would armies have to stand idle? That rarely happens when a war starts. By the 1940's, armies could fully mobilise from virtually nothing within 2-3 weeks & go straight into combat.
4. For those who argue this means the S.U. is not "surprised" by the attack .
The "surprise" is myth. Many sources told the S.U. a major attack was pending AT LEAST weeks before it occurred. E.g.'s; The British; the Rudolf Hess flight to Britain; Polilsh resistance; etc. No surprise: only Soviet government & military incompetence from the highest to lowest levels. The "West" was not surprised on 10 May 1940 when "Fall Gelb" was launched. That did not change the result by one iota.
5. For those who argue this alleged "new" forewarning would make a significant difference in S.U. preparation & troop deployment.
Allegedly, S.U. troops near the borders were aligned in an offensive posture. To change to a meaningful defensive posture would need prepared defenses in which to deploy. Where were these? Not the mythical "Stalin Line." The Wehrmacht penetrated that "Line" in true Pattonesque fashion ("Like s... through a goose.")
6. For those who argue the Red Army could simply fall back & lure the Wehrmacht into some devious trap.
The S.U. had just occupied eastern Poland (now Belarus) in Sept. 1939. It seized the Baltic states & Bessarabia only about one year before the attack. It had not yet completed its railway conversions, & certainly had not fully coerced obedience in the citizens of these areas. With each un-compelled backward step the Red Army took, even if it attempted "scorched earth tactics," it would allow the Axis easier access through those areas. This would provide a thousands of volunteers to the Axis armies, & freely give up those areas & that part of the S.U. that had the best & densest roads & railways as well as many seaports. This would greatly ease Axis supply & troop movement. Meanwhile, it would be retreating into, for the most part, a quagmire of ever-fewer roads or railways, & no seaports or ships by which to move large numbers of troops quickly.
Despite the wisecracks about the Axis "horse-drawn army", it used more trucks & tractors than the Red Army of 1941-3 & could move more troops further & faster. Axis forces, especially the Wehrmacht, were generally much better educated, led & trained, & much more skilled at operating independently. Even in 1945, S.U. units - especially armour - needed good leadership to do even minor tasks. A strategic withdrawal needs good leadership, good communication, & strong discipline. But the 1941 Red Army still suffered severely from the officer corps purges of 1937-8. Many S.U. units might lose coherence in a rapid retreat, & huge numbers of troops would be intercepted & destroyed. In such a situation, the S.U. might be apt to lose even more troops than it did historically by standing & fighting. Also, while the S.U. lacked the road/rail net of Europe, it had far more mobility infrastructure than 1812 Russia.
7. For those who argue the Axis would not be able to add any more troops to the attack.
Why not? No war with Britain means only about one corps would be needed to secure Narvik, Norway & the Swedish ore link against possibly S.U. attack. The rest of the garrison - at least 200,000 troops, could go on the "Barbarossa" order of battle. Ditto for Denmark: another 20,000 troops. Evacuate France south of Normandy, adding Armee 1 & Armee 7, etc.
I could go on, mentioning MANY other aspects of how I see no way the S.U. could survive in such a situation, let alone "win." But none of this will make the slightest difference ot the convinced.