Modern bullets? I take that misnomer to mean any bullet made the past 15 years that expands at standard velocity for 9mm and meets FBI penetration specs.
The poster above me mentions vehicle body penetration and the metal panels of the car have become thinner and aluminum in some cars. The windows are the same, but it's not all about vehicles, but more barriers. I've yet to see evidence that 9mm penetrates barriers better than .40 or .45 even with "modern bullet design."
Several years ago when I was a .40 fanboy, I would say it's the best handgun cartridge, but as I've spent years looking at data and videos, I can conclusively say between 9mm, .40, and .45, when it comes to effect on people, they're going to have the same result if you hit the person where you need to to stop them. I still lean to .40 caliber because it is a larger, heavier bullet and that makes a difference when it comes to barrier penetration.
Does that mean I find myself needing to shoot through doors, wall, windows, etc. often? No, but if I need to, I like knowing it's going to be more effectively done with .40 than with 9mm.
The magazine capacity thing is silly because reloads are so fast with semi pistols that having two or more rounds in a mag isn't a big enough difference to matter. Having said that, I love 9mm carbines that use the 30+ round Glock magazines.
"So if mag capacity doesn't matter, why don't you like .45?" Price. .45 ACP is expensive compared to 9 and .40. Were .45 prices the same as .40, I would go with .45 instead, but 5 cents more a round for FMJ is expensive, prices for .45 JHP reach ridiculous levels compared to .40.
For a full size pistol, like a Glock 17/22, I think 40 is better than 9mm, but in a single stack or subcompact pistol, the 9mm is better. Bigger gun, bigger caliber and smaller gun, smaller caliber. Pretty simple.
Everybody tries to find the one caliber to do it all and that's wrongthink. Each caliber and cartridge has it's strengths, you just have to use them in the right gun. You don't put a .22 in a Desert Ealge, it's a waste, just as you don't put a .500 Magnum in a J-frame.