This literally just occurred to me after talking with my little brother about Fujitora's Observation Haki. Fujitora is a blind man, so he needs to use Observation Haki every moment to perceive the world. Katakuri uses advanced Observation Haki all the time. There's a common theory that Big Mom's durability comes from constant usage of Armament Haki.
So why do Haki reserves need to exist? We don't even see the concept outside of the first few uses of Gear Fourth or Kaido. And it's not as if Haki is life energy, unlike Nen. IMO if Oda wanted to stick with the concept of Haki weakening throughout a fight, he could've written that Haki can fluctuate depending on mental condition. It's not like we don't have precedent for this, considering future vision needs the user to remain calm.
So why do Haki reserves need to exist? We don't even see the concept outside of the first few uses of Gear Fourth or Kaido. And it's not as if Haki is life energy, unlike Nen. IMO if Oda wanted to stick with the concept of Haki weakening throughout a fight, he could've written that Haki can fluctuate depending on mental condition. It's not like we don't have precedent for this, considering future vision needs the user to remain calm.