The 9mm is just fine as a self defense round, much depends on where the bullet is placed. I don't know what you are talking about regarding the 9mm in WWII, the 9mm FMJ round that was used did not transfer energy all that well (no different than other FMJ rounds) and certainly didn't expand. I worked as a Medical Examiner's investigator for three years, a few decades as a Law Enforcement Officer, and as an investigator for the local Coroner's office for the last 12 years and have not seen any "missing" hearts by people shot with any hand or shoulder fired weapon. Maybe with a Panzerfaust or bazooka but not a rifle or pistol. Both the 9mm and 45 ACP are noted for penetrating to the point of exiting in FMJ loadings; especially the 9mm. In some of the other so called "expanding" bullet styles, there is a good chance of overpenetration too. It really pays to look closely at one's ammunition expectations and buy accordingly.
As for penetrating bocy armor, the 9mm is much more likely to do so than the 45 ACP due to its smaller frontal area and higher velocity. Slow and wide is much easier to stop than narrow and fast which is why one needs additional ceramic or metal plates to stop rifle rounds when wearing a ballistic vest.
Law enforcement often goes for what is cheapest, what the miltary uses, what the least proficient officer can use, what "studies" have shown to be best, what the city council/county commisioners want, and a myriad of other reasons. If they wanted to use "what doesn't come out" they would go with a 22 LR. The 9mm can be handled by most officers with little training, is cheap to shoot, is "what the military uses", and is adequate for the job. Many are going to the 40 S&W in the mistaken idea that it is "much better" than the 9mm. It isn't greatly better though it does cost more and with its added recoil, can be harder to shoot as well for many officers. Its what I carry off duty and as a back up weapon but would not feel any less capable with a 9mm.
I've never been shot with a 9mm but I have been shot; you are right in not wanting to go through that experience. I will say that regardless of what someone uses, its where the bullet is placed that is important, not with what. A peripherial hit with a 45 ACP will not stop a person, particularly if they are determined to carry out their actions. A well placed shot with a 9mm is going to cause the person to stop their actions more often than not. I prefer a larger capacity handgun due to most aggravated assaults and other violent crimes are now carried out by 2 or more assailants slightly more than 50% of the time. In the old days of revolvers, the police had a hit ratio of about 35% with their handguns, meaning it took on average three shots to hit the assailant once. And that shot may not even faze him, let alone stop him. Add another attacker and a poor or non-effective hit and you could be in trouble. I prefer a few more rounds at hand and feel at least 10 rounds is sufficient in a gun. If you want to carry it, more is not a problem as it is better to have and not need than yada, yada, yada. My primary concealment gun carries 13 rounds and I keep another magazine on me. I frequently carry a "Chicago reload", i.e. a back up gun too. My duty gun is in 45 ACP but a lot of that is "machismo" more than any great advantage; in fact I think it is a little less effecient when comparing to its effectiveness on finishing off deer hit by cars. The 9mm penetrated deeper and seemed to put deer down better on body shots, the 45 often seems to need multiple hits.
I personally feel the 9mm is toward the bottom end of the cartridges to use for self defense. That is not to say it is barely adequate but it is one of the smallest I would suggest or others to use as a primary defense pistol. It also isn't all that far from the top, with the best defensive loadings, it surpasses many of the typical loadings of larger rounds.