I would say the main difference with this viewpoint is it applied in PF1 to around lvl 7 to 9, then reached a point where hit point attrition wasn't a concern. Whereas in PF2 hit point attrition isn't a concern from level 1, though low level parties drop so easily combat healing is easily exhausted and the day ends quite early.Absolutely. HP fluctuates wildly in this game and needs to be viewed much more as energy, morale etc than physical injury.
If you need HP to represent injury to a much larger degree I'm not sure I would recommend PF2 at all. Wild damage swings is inherent to the system, and I would not try to mess with this.
Adding attrition therefore would be challenging, since the game already can take you from full to zero hp in a fight that isn't even supposed to be very hard. And that assumes you enter nearly every fight at full hp!
Making the game run on the assumption heroes will still press on at two thirds or half hp basically requires you to throw the encounter guidelines out the window and generally forget about featuring monsters higher level than you. (Which of course is alright. Whether you face the Owlbear at level 3 or level 6 doesn't really matter after all)
It seems our experience differs in the game. So far we haven't been able to handle more than 5 encounters of moderate to hard difficulty in a day without running out of combat healing and high level spells. We can usually handle 3 to 4 single encounters in an adventuring day from about lvl 1 to 4 depending on time between encounters. A low level party has a very hard time doing a series of encounters.
I don't recall PF1 or 5E allowing fewer or more encounters per day. I know by high level in PF1 the party never had to stop. By about lvl 8 to 11 the party was so powerful, overpowering, and potent they could usually get back into action quite quickly with tons of cheap clw wands, a well-built healer, and cheap pots along with other abilities. And in 5E the party so over-powered their enemies if using feats, multiclassing, and magic items they rarely had to slow down. PF2 uses combat healing and spell attrition from level 1 to 20 it seems.
Suffice it to say I understand what you're saying as far as the rule goes and the potential for limitless play, but in real game play I have not seen unlimited play occur. Sandbox play has almost always been a cake walk unless you're encountering a series of encounters like a fort or something. In my opinion PF2 feels not much different in actual play than PF1 or 5E as far as attrition goes except the method of attrition stays pretty static across nearly all levels.