You gave a consequence of the rest but not why that consequence matters. Look at the next order effects of sayingI don't really understand this statement.
The group almost certainly cares about accomplishing whatever objective they set out on. Let's say that objective is to bring in a certain fugitive. They learn the guy is at a certain location. If they choose to rest for 8 hours (to be at optimal strength) before going to get the guy, they won't care that he's gone by the time they get there? And is that considered railroad levels of fiat?
Or if the group is moving through known hostile territory. It should be a real issue/discussion whether they hurry through in a weakened condition or plunk down to rest and possibly encounter hostiles (let's ignore tiny hut, as that's a different issue/discussion).
8 hours is not some trivial, inconsequential amount of time. Though if it's not enough there are options (longer rest requirements, only having a long rest in specific "safe" areas) which work for many DM (and yes 5.24 cut those options from the DMG, to the dislike of quite a few).


The quest fails? And? This one only matters if the players believe the gm will Walk away.
Other NPCs give the party gets cold shoulder from npcs? Again so what? They can sleep in a ditch and be perfectly safe recovering just fine. Magic item churn was stripped away and that's power was baked into the baseline class with everything tuned to starting gear level 1-20.
More monsters join the fight? Again... And? Are these the monsters
mearls noted PC's exceeding expectations by 5-6x?
Additional monsters are overtuned by 5-6x? Don't give exp? Etc? Now you Are back to the gm jumping in the line of fire wielding pure fiat to soak up the heated player ire for the failure of that ruleset to matter when it comes to providing gm & players with credible incentives that make taking another rest into something other than the most optimal choice actual incentives
Without ithe gm invoking fiat or players simply choosing to care, tell us what you think makes that sacrifice actually matter enough to make another rest not be the most optimal choice and it's almost certain that it really won't matter as much as implied