You don't know that. Pence's 7th shot might have been a hit. He was never able to make a 7th shot. Alleyn also was not able to make a 7th shot, and even when Kness picked up Alleyn's gun, it was empty. If it had started with 7 it could have had one left.
They emptied their guns and they failed to reload. Having 7 chambers doesn't result in a requirement that all 7 be loaded before the gun is loaded and able to fire. People often practice loading 2, 4, or 6 from a speed strip or loose rounds regardless of whether their cylinder is 5, 6, or 7 shot.
In studying gunfights, the observation is commonly made that people most often fight with what is in or on the gun, and especially for civilians, reloads are very uncommon. I've never heard of someone complaining that they brought too much ammo to a gunfight. The question, "Can I have more capacity?" is a question that is very often asked and answered by a great deal of innovation in gun design. You suggested a higher capacity cylinder that is not larger and is actually lighter rather than heavier is something that's not been asked for. I don't agree, for all the reasons I've stated.