the handgun is just your temporary means toward getting to your long gun. Rifle or shotgun.
the 9mm is fine if its you and the bad guy 10 feet from each other standing hollywood style.
If you need to put rounds thru things, hit multiple guys, and you need a leg or gut shot to take them down... then I'll take a 40 or 45 over a 9 mm any day. You may only get to wing him if you're falling over a table to prevent him from shooting you - having that ideal shot at head and upper chest isn't always there.
My little bro is an excellent shot. He had two armed bad guys come into his pharmacy - two had guyns drawn, less than 10 feet away. He had to sacrifice accuracy - He was sprinting backwards over junk, watching 2 guys at once. In a standing match, he would have killed both (with his glock 45) easily with headshots. he ended up hitting one in the gut and the other chose to exit. Shit is crazy in real shooting. big calibers rock.