I meant to respond to this earlier, but have been distracted by IRL things.
Frankly, Lanefan, this position comes across as naive at best; you are assuming that because someone does something, it necessarily must be the most fun they could have. That is simply, outright false. We can cover it with a thought experiment: Imagine you are playing Qhess, which is just like chess, except that it has an automatic win button which only activates if the player captures no pieces after playing for exactly one full hour. The optimal choice, then, is to sit there and do nothing for exactly one hour and then slam the instant-win button; by definition, you have captured no pieces (since you haven't moved!)
The problem here is exactly what I said earlier: the difference between task and outcome. The game tells players that some outcome is valuable, and thus they will pursue it. The whole point of telling the player something is valuable is to get them to want to pursue it. But, for a game to be a game and not a puzzle (or other non-game things), there must be multiple tasks which could potentially lead to that outcome. Hence, the player is encouraged to pick whichever task is the most effective at producing the outcome. But the outcome itself is, generally speaking, not much fun, for the same reason that "cross the finish line" in isolation is not a particularly enjoyable action. It is the process of getting to the finish line that is, generally speaking, where the enjoyment occurs.
Hence: Something which short-circuits that process, which lets the player completely reject the challenge, really does damage the fun of play. It turns the process into something trivial and boring.
Players will optimize the fun out of a game if they're given the chance to do so. Not all of them, of course; but most of them will. It is human nature to want to win, and if one must endure tedium, even outright unpleasantness in order to win...most players will do it.
I mean, there's also a third way, which has none of the problems of either of those, and instead gets the players to want to engage with the actually enjoyable gameplay-process of the game in question.
That is, write the game so that engaging with the process of gameplay, engaging with risk and challenge, IS the most optimal path to victory. Then, the folks who would already be doing that because they want to aren't punished for refusing to push the "I win" button, and the folks who prefer to optimize will literally optimize for an enjoyable experience because that's what is maximally effective.
Hence why I desire games where it doesn't matter which direction you look at it. Choosing to do the roleplay that makes sense and offers a fulfilling experience leads to mechanical benefits and better gameplay; choosing to seek mechanical benefits and better gameplay leads to roleplay that makes sense and offers a fulfilling experience. Games that fix the problem of optimizing the fun out of the game by making optimization indistinguishable from choosing to have fun with it.
One bandaid solution for this is using XP as a balancing tool ...
I think 4e did this, but am not sure:
The more encounters your have between long rests the more you get (like in multipliers).
So, to put that into 5e as a quick and dirty solution is, when ever you take a long rest, all XP you got till the start of the Level gets halved or a specific amount of XP gets removed (adjusted by level). By RAW except for the first levels, you need 2 to 3 adventuring days to get enough XP to gain a level - so you adjust the XP Reduction and XP you give so that that stays true. But when you take more long rests, Leveling up gets harder.
So for example:
You need 1000 XP for the next level.
Normal would be:
400xp trough adventuring - long rest - 100xp = 300xp, 400xp - long rest 100xp = 600xp, 500xp adventuring- long rest - 100xp = 1000 XP level up.
3 adventure days to level up.
Now, with the 5minute adventure day:
100xp adventuring- long rest - 100xp = 0 XP gained total.
So they will not level up at all.
More careful adventure day:
200xp trough adventuring - 100xp for long rest that's 100xp.
So they need 10 "adventuring days" to level up.
Like, that is a purley mechanical incentive with no inworld representation ...
Maybe the other way would be better ...
Getting a bonus for more encounters.
So of you only do one encounter, your XP is multiplied by 0,5. If you have two encounters the XP you gain is multiplied by 1, with 3 encounters with 1,5, 4 encounters with 2 and so on (0,5 multiplier per encounter more).
That sounds more positive.