I'm A Banana
Potassium-Rich
None of that really invalidates the idea of HD being your long-term resource and HP being your short-term resource.Rituals aren’t free, however. They just use a different set of resources (rather than spell slots) that could still be drained over time. It’s 70 gp of reagents a casting for 12 hours, after all, but I can see how it would need to be adjusted to fit in a more exploration heavy campaign. One thing to do is limit how much reagents are easily available, maybe even pushing PC to break down lesser magic items into residuum. How did you party achieve consistent 40+ Arcana checks to get those flying steeds anyway?
One thing to note that 5e does that could be applied to a 4e system is not automatically recovering every HD on a long rest. That could add to long term challenge if you only recover your CON mod +1 or something during a long rest (I’d give Bards a feature that lets them boost that recovery with a song) or make a full recovery contingent on an Endurance check with penalty based on how rough you’re roughing it.
They really REALLY are not the same thing. Healing Surges were the back bone of pretty much everything healing in 4e. Healing Word? Needs a healing surge. Healing potion? Needs a healing surge. Surgeless healing was incredibly rare and usually limited to slow regeneration or high level Cleric Prayers. And Healing Surge meant that every instance of healing had roughly the same value for every class as it was ALWAYS 1/4 your max HP instead of leaving it to chance. Healing Word was good at every level because of that scaling and a potion that could grant you a healing surge was always useful. It also created a hard cap on how much healing could be used in a day for each characters, creating a universal daily clock. Yes, some class had more surges, but those were the classes that usually took more damage so it all balanced out pretty well in the end. Furthermore, the number of surges you had didn't grow with each level. You started with a class specific number +CON mod and it only increased when your CON mod would. Their value would improve with your max HP, but the number was stable and you started with a decent amount, this meant the number of fights you could endure in a day was fairly stable from level to level based on your role. I think it's stupid a Fighter and Wizard both only get 1 HD per level.
Healing Surges also had uses in rituals and could be drained by traps or environmental effects.
Healing Surges were intertwined with the system in ways Hit Dice can only dream of. The were literally a corner stone of it.
Sure, there were a lot more things you could spend that resource on in 4e. And, spell slots are another long-term resource in 5e that didn't exist in quite the same way in 4e (they're analogous to daily powers, but the comparison isn't quite clean), so some supplemental healing might be included in that long-term resource, optionally.
But the basic idea of HD being a long-term resource and HP being considered a per-encounter resource holds true in 5e.