they have in reserve at least 2000 of them.
More than 5000 are in the deep store and you are getting those. Refurbishing and modernisation process for those is slow.
Pre-digital models are still better then Pt91
They are not, if we look at the 80's M1A1, the main advantage would be better turret armor, better ammo (if ammericans deliver the modern stuff) and higher road speed. FCS would be actually better on PT-91, better off-road mobility and don't get me started on the fuel consumption (no power generator in older models).
it takes months to fully spin up military production when the demand finally hits.
That was in the Cold War, but now, with 30 years of aggressive de-industrialisation it would take years in the best case scenario. Keep in mind that thanks to the magic of mangerialism, we have massive lack of industry skilled workers with generational gap looking more and more abyssal. And this is not an issue that can be solved overnight, for the last 20-30 years the physical work, even skilled one, has been downright vilified, if you did not go to uni you were practically a subhuman and despite efforts of people like Mike Rowe, the effects are now well entrenched and it will take generations to undo the damage, as it would take for us to off the debt driven economy and the elites are having none of it. The trades training courses are too few, far from filled and at best half of those taking them are fit for work they learn for.
And let us not go into whole sectors of manufacturing being exported overseas.
How much ammunition was disposed of by NATO after the Cold War ended?
Most of it, some of it was sold to third-world countries, but much of it was destroyed. Nominally the NATO countries keep the 90 day stock for their much reduced militaries, I think the artillery normative for tube artillery is one combat load per day, which by the Ukraine war standards is insufficient.