It's generally easy to rationalize punishing players for deviating from the expected day length (or week length, if you go that way), yes... generally...
...But, If the players are exploring an ancient tomb, untouched for centuries, that only they know the location of, and they decide to retreat from a powerful construct guarding one of the inner chambers, re-seal and cover the entrance, rest up, make a new magic weapon capable of harming said construct, and come back to dispatch it later, well, that's just 'bout Smart Play, i'n'it?
You could rationalize a punishment there, and if you don't, the players may realize they can slowly grind away at the challenges in the tomb, methodically, one challenge and one long rest at a time, which would render the whole resource-attrition balancing act moot. But, if you do, they players may realize you're forcing things and their decisions don't really matter.
Really, there's three things being discussed here as if they were one thing, because, in D&D, they're all affected by resource mechanics, hp/HD and Slots, primarily:
- Pacing: Stories need it, games need it, life has it whether it wants it or not. Things happen at different paces in different times places and situations, sometimes frantic, sometimes methodical, sometimes hurry-up-and-wait. A lighting raid, a methodical exploration, a dangerous wilderness journey, a traditional dungeon - time pressure varies and challenges come at you at different rates in each.
- Stakes: Your characters are risking their imaginary lives and you're managing their abstract game resources, for a purpose. To gain levels, at minimum, that's built-in. To gain loot is not as built-in as it used to be. You can even be trying to accomplish something in the context of the world, rid the land of evil monsters, save innocent lives, make a name for yourself, etc... There will generally be risks, and there can be trade-offs and unintended consequences to minimizing those risks, in-fiction, hard-coded, or meta-game.
- Balance: A cooperative game that presents choices that are strictly inferior is a game being dishonest with it's players. The players either get wise and powergame making the game less challenging or get suckered and are a drag on the rest of the team. D&D is such a game, but it has a strategy to ameliorate it: the Adventuring Day. Classes are given unequal resources, but players are forced to manage those resources over a carefully calibrated series of challenges that strain them to the point that those with superior resources run our or must conserve them, putting in poor performances relative to the higher baseline of the classes that have fewer resources.
An issue we run up against is that most solutions that get floated don't address all three of those. They focus on just a single issue, and leave the other two out, or even make an issue worse.
Like, obviously, just laying it on the DM to force grueling enough adventuring days may reduce class imbalance (3), but it gets in the way of the DM pacing his campaign as he might like (be that driven by realism or drama or something else), and it artificially limits the players' agency to set Stakes by focusing resources on a short day.
For instance, an entirely encounter-focused game, like the 7th ed of Gamma World, where you recover all your hp and other resources after every encounter, means any pacing is possible, and resource-balance is a non-issue, but what are the Stakes for preparing for a tough battle or blowing through several? Nuthin' every battle you go in exactly the same.
There are no stops to pull out. Similarly, the 13A solution (quick rests every encounter, full heal-up every 4th encounter) solves (1) and (3) handily at the cost of (2) - 13A adds it back with the option of a 'campaign loss' to full-heal-up early, but I doubt that's entirely satisfying.