When did it become absolutely impossible for Germany to survive in World War II?

Germans did have some successful training exercises with the Rhine barges but of course, these were conducted under good weather and right next to the shoreline. Kinda doubtful whether that means the barges could have actually carried troops successfully across - though the Channel isn't very stormy in late July - early August. Of course, it's again doubtful whether the Germans could have been ready to get their first wave across in late July. In fact it's impossible since Hitler historically gave the order for Sealion in mid-July - but if we change things around and Hitler gives the order in mid-June, when the French collapse was becoming crystal clear, maybe they could've managed the timing.

That still leaves unanswered the question whether Luftwaffe and Kriegsmarine could have kept both RN and RAF out of the landing zones long enough.

As for casualties in BoB, it's noteworthy to remember that afterwards, when the roles were swapped and RAF started sweeps over France and most of LW was in the East, the casualty ratios got turned around as well - British suffered way more losses than Germans did AND of course lost most of their downed pilots. This doesn't help the Germans invading Britain, of course, but it puts the casualties LW suffered in the right context - RAF fighter pilots were not magical creatures after all

Failure to take Moscow was the end really.

The Russians might have fought on but without the infrastructural hub of Moscow; their ability to supply/engage/move troops/materiel across such a wide front is dubious at best. The best Germany could have hoped for was some sort of negotiated peace.

Even if Germany had defeated the Soviet Union, she possibly didn't have the manpower to effectively govern/suppress the vastness of the USSR. It's really just a matter of time then until Aircraft Carrier One (UK) staged the first of many nuclear strikes into Germany.

Germany lost the war when they declared on USSR and started a two front war, voluntarily. Utter madness.

They had already conquered a continent (Europe), had they won not single square inch of ground from then on, Hitler would be the greatest German leader that ever lived - GROFAZ indeed. Bismark united Germany and took bits of Poland. So what. Hitler did that, and annexed Austria, Czechoslovakia, Netherlands, Beglium, Denmark, Norway, Luxembourg, Yugoslavia, Greece, and France. Every nation remaining indeependent was either allied to Germany or trading with them on favourable terms.

Lebensraum? They had a vast amount of it already. No need for a drang nach osten.

They take all the land they needed for their own people by evicting those in the lands they conquered. They had access to all the mines and natural resources of the european continent, and had limitless amounts of slave labour to exploit.

They even had a marraige of convenience with Stalin. Coal for Oil or Iron? Swap. Manufactured items (built by slaves) for Russian Oil or Steel? Deal.

What a rational leader would do, at this point, is wind back the war footing, and set up the economy to produce the sort of goods that will make the Deutsche Volk truly believe they are living the facist dream. Volkswagens etc. Of course we're not advocating disarmament but simply setting up a solid defensive line against the Russians in occupied Poland, to keep Stalin honest, and steadily building up the Kriegsmarine.

At that point, Germany was only at war with the UK who were indebting themselves to the eyeballs to sustain a war effort that was going nowhere.

Yes, the UK and the USA together, managed to navally assault and liberate occupied Europe, with 80% of the Heer in the east fighting Russians and with 3 years to prepare landing craft etc.

Realistically, the UK could not do this on its own, with the whole wermacht arrayed against them. All they acheived, up till D-Day, was to fight (mostly Italians) in the economically unimportant North African theatre, and to send fleets of very expensive bombers, that in the early years, usually lost more trained aircrew in each raid than they managed to acheive in terms of German civilians killed.

At the end of the day, we were dependent on Uncle Sam to keep writing cheques just to stop us from being forced to make a white peace, and we needed help from both Sam and Joe to defeat the german army.