Good Point Jimlock. And I'm glad you brought it up. If Defensive ability is a part of the abstract equation of hit points, then attacks would be also. The better you are at attacking, the quicker you bring down opponents, resulting in less "damage"...
So, I don't see it that way.
Even sticking with the D&D definition of hit points, I don't see it that way.
How hard to kill, not how hard to hit.
Punishment accrual, not avoiding punishment.
Punishment accrual, and the energy expenditure required to mitigate the amount of physical damage you take (rolling with a punch, taking it in the arm rather than the head, etc.). But you still take damage. Hit points aren't about "avoiding damage".
That last sentence may make it sound like it includes defensive ability. But if defense is included in HP, then by that definition "attacking" would be also. Your skill at attacking is just as important at keeping you alive in a combat situation as defense is.
Defense and defenseive progression isn't about how tough you are, or how much punishment you can take, or how much combat endurance you have, or your luck at surviving combat. It's simply the skill you have at avoiding or neutralizing attacks in the first place - without taking any damage or punishment.
Real combat training focuses as much on Defending as it does on Attacking, and sometimes even more focused on Defending. It makes little sense that characters get better at attacking, but never improve their defensive abilities except through the inclusion of Armor or Magic Items. (Of which if we accept the idea that Defense is part of HP, then Armor and Defensive Magic Items should increase HP rather than AC/Defense...

)
From a game perspective however, Defensive progression has never been a part of core D&D, and adds an unacceptable amount of complication to the game for many gamers (extra bookeeping, sometimes extra rolls, etc.). That's why I think it works best as an add-on module (like it was with 3E's Unearthed Arcana), rather than part of core. But I strongly feel that Defense Progression should definitely be included as a part of 5E (whether core or add-on module).