Rory MacDonald says hi. That fight was ended directly as a result of his nose being shattered by Lawler and it just happened in his fight before last. Are you going to question Rory's heart now too? I didn't think so.
First off, it depends on how bad the break is. The extent of it affecting or potentially affecting the fighter going forward, and how many rounds there are left to go, secondly. I've seen broken noses that have bled like water faucets, eyes pouring tears and fighters feeling very woozy due to the rapid amount of blood loss. If a fighter gets their nose broken badly and can't properly breathe out of it, that affects their breathing and can make them exclusive mouth breathers. Mouth breathers are much easier to knock out because they're relying on breathing out of their open mouth, not properly through the nose first and then out through the mouth on exhalation.
If I knew that my opponent had their nose badly broken and they're doing nothing but mouth breathing, I'd be looking to either crack their nose directly again, or even better, crack their loose hanging jaw knowing that biting down on their mouthpiece to help them absorb the shock of my punches will be less frequent than it was prior to their nose being broken in the first place. It can affect their breathing pattern, directly as a result of which their cardio, and everything from off of that; ie., their offense and defense.
Even worse, if a fighter is ignorant enough or accidentally out of habit clears their nose after the break then they'll likely end up with one working eye due to immediate swelling. It's a mistake that fighters have made before, even champions, in many sports. You now have a broken nose, maybe even a badly broken one, you're breathing much more heavily out of your mouth, and you have one good eye.
Didn't you ever watch Karate Kid 3?!
The very first thing you're taught in any respectable self-defense course is to directly target vital points such as the eyes, nose, throat and groin. Go take one, a military combatives course or some Krav Maga lessons. You're ignorant in thinking that the primary breathing apparatus being impaired isn't important enough for a fight to be legitimately stopped in at least some situations, depending on the criteria that I listed.
SMH.