It has always seemed to me that a Nazi win would end up with the collapse of Germany, either because the economic model of endless government spending would fail once they ran out of plunder,...
I am very much against "endless government spending" just like you, and as I have already stated I am very much against Nazism, just like every sane person.
But what exactly is "the collapse of Germany"?
I have already pointed out the possibilities for a country with severe economic difficulties:
- Being occupied by a stronger neighbour. This is unlikely precisely because we are dealing here with a "Nazis win" scenario.
- Splitting up into smaller entities. Germany does not have a tribal society. The separatist movements after World War One were jokes and collapsed within days. (Yes, some political entities
do simply collapse - cease to be, end their existence, become a
former entity etc. etc. With a country it's not so easy)
- Regime change, either peaceful or violent. Having won a war usually increases a government's popularity. In a post WW II Germany, both OTL and ATL, you have millions of former soldiers who have seen the misery of the people in the USSR at first hand - misery that existed on the very first day on which they occupied the area in question. The Nazi government will tell everyone "it is we who have protected you against all this misery" and there will be lots of people who will believe this, at least among the Germans. The fact that the RAF made a point of bombing civilian areas will also help Nazi propaganda immensely, again making it look like the protector against an outside threat.
It is unlikely that there are many people that will risk their lives in an attempt at a revolution or coup only because there are economic difficulties. In OTL there were Generals who considered a coup in 1938, but did not dare attempt it because Hitler was so successful at Munich. You will have the same attitude squared after a victorious World War II.
- Creaking on, as the Soviet Union or North Korea have creaked on.
And please remember that in a way Germany had been there before. The governments of the Weimar Republic inherited enormous war debts from their predecessor governments from WW I. They got largely rid of them during the hyperinflation of 1922 and 1923. Although this was far from being an intelligent way of tackling the problem, the Weimar Republic continued its existence for about a decade, and its end was far from being an inevitable consequence of war debts or inflation. And there is also the obvious fact that Weimar Germany had to pay reparations, which a victorious Nazi Germany won't have to pay.
... or because the sheer brutality of their genocidal policies towards the Slavs would cause overwhelming guerrilla resistance in Eastern Europe.
No one is doubting the brutality of the Nazi occupation. But guerilla movements were only a minor factor compared with the conventional armies and air forces that the German armed forces had to confront.
After a Nazi victory, guerilla movements are almost bound to decrease, not increase, either because the factories that have supplied the guerilleros have been occupied by the Germans, or have been bombed out of existence, or because the government of a rump USSR or defeated UK is afraid of German reprisals if they keep supplying any resistance movement in German occupied territory.
Please remember also that for a "Nazis win" scenario the Nazis must have done something intelligent in the alternate timeline, where they have done something stupid in OTL. Being intelligent enough to win WW II and then being stupid enough to "make Germany collapse" (whatever that means) is a very unlikely combination.