Gonna be honest here: the adventuring day is another piece of design that needs to go away for good.
That's a separate argument though.
With the widespread acceptance of milestone leveling, it's pretty much just there to waste valuable gaming time that could be spent on social or exploration stuff.
'Adventuring day XP' isn't there to determine the
pacing of PC levelling.
It exists as a
class and encounter balancing mechanism within the context of DnDs resource management model.
The '5 minute work day' has been a feature of DnD in all its incarnations (even 4E's AEDU system) by virtue of the fact DnD is (mechanically) a resource management game.
Spell slot, rages, Ki points, action surges, second winds, indomitable, superiority dice, channel divinity, lay on hands, XP, GP, HP, HD, magic item charges, Sorcery points, wild shapes, X per long/ short rest abilities. When you are only getting 1-3 encounters between long rest recharges, you can nova the crap out of encounters, and this favors long rest based classes (i.e. Casters).
Long rest resources are designed to last 6-8 medium to hard encounters before being expended. Short rest resources are designed to last around 2-3 encounters before being expended.
Letting players dump them all in a single encounter does away with that balance, and dramatically favors classes that are Long rest resource heavy (because those abilities are far more potent than ones that recover on a short rest). Casters can dump high level slots like candy, while still having enough petrol in the tank to magic away environmental and social challenges as well, with little to no resource pressure on them.
Now you can critique that design choice all you want (maybe all abilities should be based on a [per encounter] model, with an auto refresh at the end of each encounter - as was the case in SWSE and ToB:BoNS), but it exists.
If you run games outside of that expectation, you're going to get very different results.