That's a very silly hypothetical. The idea that either of those guns would be loaded with only 5 rounds is patently absurd. Setting that aside, I probably go for the G17 even if I only get 5 rounds, because I'm acclimated to shooting 9mm and not to .45 and therefore will most likely perform better with the G17 than I would with the G21.
Whenever I see the word "obsolete" used to describe a smokeless metallic cartridge I immediately assume the writer is going for outrage, rabid agreement, or just trying to stir up comment for no useful purpose. What people say when they've run out of things to say, yet still want to say something.
A .429 hard cast bullet going 1000+ fps does the same amount of work if it's fired from a .44 Spc, .44 Remington Magnum or from a 44-40. That's why we still have .348 Winchester, 38-55 and 45-70 trucking along in small, but effective numbers.
The limiting factor then becomes, is the chambering available in a firearm type that suits our intended use.
How are you running them? Are you running them hard? Mine run 100% to when I’m plinking at the range. Do you do some speed/panic reloads, run it hard, draw fast maybe not perfect grip? Then things start to trickle in. Honestly they still run pretty darn good if they’re built right, But those minor quirks quickly turn to a problem in a stressed situation.
Considering the amount of time, research, and effort given to make this platform work over the past many decades, makes you realize it wasn’t a good starting point in the first place. Just the effort given to the magazine changes/upgrades over the years tells you the issue with making a short tapered round feed a gun designed for a longer straight walled round.
Of course, YMMV.
No, they are toys at this point. Maybe that is the difference. Carry guns for me are Double Action. Frankly, I cannot see the 1911 as a good "serious" gun choice for most folks, although I know that is heresy. Those willing to spend a lot of time and money on a good gunsmith, and/or top notch gun, or who are willing to acquire the knowledge and tools and devote the time themselves to the finer points of 1911 maintenance are an exception.
On that "5 rounds" question that was posed, my answer would be that yeah, if ball ammo I would likely choose .45, but why would I handicap myself that way? With an intelligent ammo choice, it would not matter. 9mm, .40, .45... all fine. Start messing with distance and intermediate barriers of different substances and I might have preferences.
Regarding the question posed in the thread's title, Betteridge's Law of Headlines is pretty accurate in my experience.
I ran 9mm DW’s on duty for years, and ran them hard. I never had any issues. I have a lot of confidence in their ability to properly build a 9mm.
That being said, if I was to get back into 1911’s, they’d probably be .45’s. There are few handguns more fun than 9mm 1911’s, though.