What happens internally in Austrian domestic and wider European affairs.
To my knowledge,Prussia was also in a state of decline in Frederick the Great's latter years,especially into Frederick William II's reign.If the war continues, Austrian debt increases, Prussian intervention becomes more and more likely (so no guarantee of Austrian victory or any territorial gains at all), Belgians starting revolting due to the military focus elsewhere, Hungarian nobles are revolting, etc. Joseph's policies weren't popular and the war only sours popular opinion further. Him continuing his course and staying in the Balkan bog only worsens Austria's position in the long run (come French Revolutionary Wars, the Habsburgs will have to capitulate sooner than they did OTL due to dissent and debt. France does better in the war, perhaps to the point that Napoleon doesn't come into prominence in Italy? Anyways, Austria's stability is inversely proportional to Joseph's lifespan, especially with the Coalition Wars coming up).
Shame too, since his heart was in the right place and some of his reforms would've been quite helpful to Austria down the line. Now, if he had just been a bit less forceful and centralizing, perhaps...but just giving him 5 years extra when he's already alienated the whole nation only hurts everyone down the line.
The way I see it,Prussia's gonna have a far more difficult time replacing that army than Austria.The Austrians simply have far more resources than Prussia.I've heard similar views of Prussia as a paper tiger at this time:
"Prussian army of this period was a deteriorating
institution living by the memories of the Old Fritz with the obsolete tactics
and no good generals The Austrians, IIRC, still have Laudon "operational" and it
was proven later by Suvorov's Italian Campaign (and by the 2nd Ottoman War)
that when led properly, they could perform quite well...
the Austrians were not alone [they were allied with the Russians]and Prussia of FWII was hardly capable to fight against this combination with any
chance of success. The Russians had some of the best generals of the
time (Rumiantsev and Suvorov) and their tactics was quite close to what later
became Nappy's trademark (with less effective usage of the field artillery,
of course).
On the other hand, numerically, the Prussian army was the third largest in Europe, which could count for something.
It's tough to use those resources when much of your realm is on the verge of rebellion. Joseph II's Austria was not in a state to handle much more stress.The way I see it,Prussia's gonna have a far more difficult time replacing that army than Austria.The Austrians simply have far more resources than Prussia.