I'm not really sure where you heard all handgun caliber a are weak,
Could come from the oft-repeated statement that 80% of people shot with a handgun survive...
Hence the advice to carry a gun chambered in the most powerful caliber you can shoot well.
Compared to rifles, handguns are weak. So? The fact that there may be 80% of the people shot survive has nothing to do the how "weak" handguns are (in general) but how good medicine is today.
Back before the 1920s, (modern germ theory) and the discovery of antibiotics, getting shot, even in a "non-fatal" spot was a 50/50 death sentence. You either lived or you didn't. Infection killed far more of those shot that the actual wound did.
Big bore rounds like the .45 Colt were often more survivable than smaller calibers. Round that went all the way through were often more survivable. One plugged the holes, and hoped for the best.
Round with enough power to go halfway through were more deadly, more often, as the bullet stayed in the body, and the dirt, lint, dust, etc stuck in the bullet lube meant infection (usually fatal) was extremely common. One of the most feard guns in the old west was the .41 derringer, as its outside lubricated bullet usually would go about halfway through a man and stop, virtually insuring painful death in a couple weeks, or less.
Today, with modern medicine available within a few minutes (except in the remotest locations), if tissue damage and/or blood loss doesn't kill you outright, the odds of survival are very good, much better than at any other time in history.
And, interestingly enough, smaller caliber rounds (including 9mm fmj) have a higher fatality rate from complications than large bore rounds. This is because overall, people are shot more times with smaller size rounds before being stopped.
Not trying to start a caliber/stopping power war as that is beside the point. Individual rounds, one shot stops, and all the statistics and arguments about what is "best" have their merits, and their place, but its not here.
What I am trying to say is that someone stopped by a couple rnds of .45, for instance (non lethal hits) is more likely to survive than someone who takes 6, 8, or more 9mm hits.
Modern medicine is very good, and a lot of people shot multiple times do survive, but not all, by any means. The more times you are shot, the greater the "insult" to the body, and the more likely you are to die, despite all that can be done. So, the most "humane" round is the one that is most likely to stop your assailant, with the fewest number of hits. I'll leave the argument as to which that is to you.
Personally, I think that the "80% survive" figure is doubtful. 80% surviving AFTER getting medical attention might be reasonable, but one would have to look at the study parameters closely....
One should not use how "lethal" a round is as the metric for a defensive firearm. Stopping power is what is important. Attacks must be stopped. That is the first and foremost issue. If the attacker dies as a result of being stopped, they die. If they live, they live. The only important thing is that they are stopped from harming you (or others).
Sure, its a game of words, but word matter, especially to the legal system. One shoots, only when one has to, to STOP the attack. IF you "shoot to kill", or even "shoot to wound", there is a good chance you will well treated by the legal system. However, you will be around to be judged, which is always better than the alternative. If you attacker is able to be judged in this world, fine. If not, they will be judged in the next, which is also fine by me.