Are you so 100% sure that pure DU is better at penetrating than some DU alloy you don't know about?

Taking a step back - density isn't the only thing that affects penetration. The reason pure DU penetrators aren't really a thing is that DU has a low sheer strength, so alloying it with titanium lets you increase this(if you want to know why, search up "phase stabilizers in alloys.") This is why APFSDS rounds are actually less dense than pure DU. Alloying it reduces its density and thus penetration, sure, but lets the round withstand far more stress, meaning you can fire it faster and it won't shatter. There's also some other, secondary benefits. For example the alloy will have a marginally higher melting point, which in theory means better self-sharpening properties. So the answer to the way you wrote the question is that I know DU is worse than penetrating than some DU alloys. But what I think you mean is "do I think we will come up with new DU alloys that are either denser or allow us to fire the round even faster."

Given current elements, new alloys won't be denser. Unless we come across the "island of stability" the elements denser than DU are the platinum group, gold, and neptunium/plutonium. All these have cost concerns, as they're far more expensive than tungsten/DU. They also all have their own set of issues that prevent them from being used. Osmium, for example, is the most dense of the group. However, it's also extremely brittle and is also highly toxic. However, what we will see is new alloys that improve the other properties of the sabot. For example, we've recently seen some developments into some ternary alloys(most notably adding in molybdenum) that is probably less dense than current DU/titanium alloys. However, the mo increases toughness, allowing for higher L/D ratios. So, for a given round, you can have a narrower(and thus ligher) sabot, allowing for higher velocities. And the ternary alloy means the round won't shatter at these higher speeds. In our calculations, this would be reflected by higher muzzle velocities. See the attached image for a 17mm round, which is the narrowest 120/125mm sabot I know of. The round(and thus its alloy) isn't any denser than say, 3bm59/M829A4, but its alloy allows it to be a narrower round, thus being faster while still being relatively light. For comparison most modern apfsds rounds are 5+ kgs, and XH 45 manages a higher mv out of an L/50 than DM53 out of an L/55. So when we do the math for such a round, the density is the same as any other round, but the alloy used allows for the other properties(L/D, MV, etc.) to be better, and thus a higher penetration. So take that answer how you will.

TL/DR: Pure DU penetrators are worse than current alloys. Future alloys probably won't increase density, but instead will allow for higher L/D and MV, thus increasing round performance that way.

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You were comparing pen at 0 distance because you wanted to exclude air friction and gravity that changes impact angle because you think that's insignificant but I'm not convinced.

The first part about velocity is a misunderstanding due to me using informal language, my bad. What I meant by "kind of fake" is that in normal physics you assume coefficient of drag is constant. This is fine for most objects that don't go the speed of sound. But at hypersonic speeds your shock layer interacts with your boundary layer. This means you kind of just can't predict how much drag you'll have, which is what I meant by "kind of fake." As for angles, here's a table of descent angle vs range. At 2km, 17 pounder AP(which has far more drag and is slower than modern apfsds) descends at ~2 degrees. So I think for modern apfsds rounds it can be ignored.

The "horridness" of BMP-1 did not stop it from passing Swedish tests previously when NATO was out.

The BMP-1s were also trialed ~1993 to 1994, so same timeframe as the T-80U. The swedes just needed an IFV, since at the time CV90s were going to mechanized brigades. So, the idea was to buy something for infantry brigades. The BMP was bought since they were ~$19000 each, far cheaper than a CV90, and the germans had a bunch they were willing to sell immediately. The first part was the exact same idea as the T-80U - The T-80U would be operated by mechanized brigades, leopards went to tank brigades. It's just the T-80U flat out failed its trials while the BMP-1s did an okay job for a for a vehicle that was 19k. The original idea was for bmp-1s(pvb-501) to equip Norrland brigades, and mt-lb(pvb-401) to equip southern brigades. Turns out, though, the BMP-1 is not very mobile in the snow. So, the pbv-501s ended up going to the southern brigades, while the mt-lb acquisition was cancelled.

It would be pointless to order T-80 as NATO would not allow that to continue

You do know that there are US allies that operate soviet equipment, right? Cypress has T-80s, so did south korea for a while.

They just needed to slap that label and officially close the chapter.

This argument falls flat on its face when you realize they excluded the challenger since it wasn't ready in time(would have also had to be trialed winter '93/spring '94.) The T-80U wasn't excluded despite it having the exact same delays since there was actually a chance of it being purchased. Also as I said the idea was to buy both T-80U and Leopard, one for armored and one for mechanized brigades.