The rules are pretty much clear.They indicate whether or not you are effective in combat, but they do nothing to measure how effective you are. Someone at 42/50 is not any more effective than someone at 34/50, or someone at 42/100, or someone at 7/7. We know this to be true, because the rules tell us this.
What Hit Points do measure, explicitly, is when and how badly you are hit. Someone at 34/50 has been hit twice as many times, or twice as severely, as someone at 42/100. Again, we know this because the rules tell us this.
Be you at 100% HP or 1%, your combat effectiveness is full. No loss.
HP represents luck, gods' favor, experience, plot armor and have nothing with how you fight. Your level and Proficiency bonus are what measure your combat effectiveness.
HP are not there to show how much physical punishment you can take, but how much you can avoid!
Take this scenario with Bob at different levels.
Bob the 1st level fighter is fighting an orc. The orc swings its axe for 13 points of damage. The axe cleave through the armor. Bob falls down, uncounscious. Will it be a fatal wound? Who knows. Billy the Bard is a bit far to heal...
Bob the 2nd level fighter is fighting an orc. The orc swings its axe for 12 points of damage. Bob barely parries the blow to his head. His experience saved him that injury. He is a bit tired, but he can press on the orc. Bob hits the orc for 13 points of damage, the orc barely parries with his shield. Bob wins the next initiative, and hit the orc for 10 points of damage. Now Bob kills the Orc with a clean strike to the heart. Billy the bard is still stuck with an orc. For some reason, the Orc keeps missing Billy but the orc is enraged beyond belief. "My mother was not a GOBLIN!" can Bob hear the orc. Bob rushes to Billy's help.
Bob the 20th level fighter is fighting 6 orcs. They all hit him for 12 damages (that DM is lucky isn't he?) Bob takes 72 damage but Bob parries all attacks. A lesser fighter would have been cut in pieces but not Bob. Bob feels that it is urgent that he does something. Bob did not see the arrow that struck his back for 8 damage as the armor deflected the arrow. Bob attacks the orcs, fell four of them, Action surge, kills the two others and rushes to help his friend Billy the Bard in a deadly lock with 4 more orcs. Bob kills two of these with his remaining attacks. Ho Bob does have a few scratches and bruises. But Bob is mostly fine. Nothing dangerous to his health was done. But better get some healing to be sure (or a short rest or whatever). And there are two more orc archers to deal with...
Here you are. You saw Bob at various stages of his career. Only in first level did the axe's swing put his life in danger. The other times, plot armor, experience, luck and gods' favors protected Bob. Bob in the two last examples is bruised, hurt and might even be bleeding a bit, but nothing serious was done to him. That is why his combat effectiveness isn't altered.