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Lets say you have the same two pistols for the ammo above. Both pistols are polymer and have a 3 in. barrel and are made by the same well known gun maker. If you put 10 thousands rounds a year in both for lets say 5 years am I correct in thinking the .45 pistol will suffer less abuse than the 9mm because of the pressures of the 9?

Beats me, but I know the ammo cost for the .45 is significantly higher compared to the 9MM.

Would be a great scientific test.

In the gun world we hear all sorts of opinions---some based on science---some maybe not so much. There is a ton of anecdotal evidence (I did this and this is what happened) but always good to see if any scientific testing has been done under rigorous controls.

Not an engineer but I think that each is designed to handle it's respective pressures... so even if the same platform, there are some differences in the "beefiness" at critical areas... or in the case of others, they are so overbuilt that the question is moot.

Again, just a simpleton's opinion.

If you have enough money to put that many rounds through either one, you could easily buy a decent 1911 or three Glocks or so (~$.15/round on the very cheap end, 10,000 rds comes to $1500) over that time period.
But to your question, I believe that longevity is not necessarily predicated on caliber as much as it is on maintenance and upkeep, at least with modern reputable guns.
As far as pressure etc., each caliber gun will be built according to required pressure specs, so unless you are shooting 10,000 rds of +p+ in 9 as opposed to standard 230gr ball .45, the difference in longevity will really be imperceptible. My opinion at least.

Although I don't believe the differences in pressure will have any effect, I suspect that all things being the same, bullet velocity might have an effect on barrel wear. I also suspect that differences in barrels such as standard rifling and polygonal, and being chromlined will have an even larger role.

The 45 will wear faster. More Muzzle energy means more powder is used. More wear is the result

If you properly maintain and lubricate the pistols, I don't think you will see much difference. I have M&Ps in both 9 an 45. The 9 has 50,000 + though it with no detectable wear. The 45 is probably closer to 30K and the same for it. I do use a good synthetic oil on both and keep them very well lubricated. I think the lubrication part is really critical to extending the life of any firearm.

I typically use loads for the 9 mm that make 135,000 power factor (147 grain bullet at 920 feet per second) and 45 loads that make 175,000 power factor (200 grain SWC at 875 feet per second). Not cream puff load but not self defense level loads either.

I'm not familiar with all brands/versions of polymer pistols made in both 9mm and .45ACP, but I've got two sets, S&W Shield in 9 and .45, and Beretta PX4 Storm in 9 and .45. In both cases, the .45 version is built bigger, so not apples to apples.

I have also had the S&W M&P in 9 and .45, and the Kahr CW9/CW45. Same thing, the .45 version is larger, so again, not apples to apples.

Unless you're thinking of a particular polymer framed 3" barrelled pistol whose 9mm and .45 versions are actually the exact same size, I'd say your question is fairly impossible to answer accurately.

My innacurate theory is that the 9mm would be shot more due to economics, so would wear out faster, though I've never worn out a polymer pistol. They all have replaceable parts.

Either way, you are spending many, many multiples of the gun cost on your 50,000 rounds of ammo, so replacing the gun would be a minimal cost in comparison. But I'd say that both guns will take a beating and at a minimum will need to have the barrels replaced, along with multiple replacements of the springs, of course.