Session length has nothing to do with the adventuring day. Correct. Most groups choose to combine session length with the adventuring day.
1) Do you have any proof this is the case?
2) Even if this is the case, that's the fault of those groups. The rules are not: 'at the start of the session your PCs resources are all fully recharged.' If groups ARE choosing to refresh all abilities at the start of every session (as if a Long rest had occurred between sessions) that's the group not following the actual rules of the game.
That most groups do not have long adventuring days. They have one, two, sometimes three encounters and then they rest.
And if you're the DM of a Group that only has 1-3 encounters before resting 8 hours, you should be using the Gritty realism rest variant. That way your overnight rests are Short rests, and you need a full week back at base to Long rest.
At the end of the day Groups that are only getting 1-3 encounters between Long rests, are doing so by consent of the DM. If the DM is allowing the 5 minute adventuring day, and it's causing problems, then that's on the DM.
You are correct, in time sensitive dungeons or areas, those spells only provide a safe long rest, but it might also mean that the PC's don't "win." But, again, I would argue that time sensitivity is not the norm for a lot of groups. And that would push my original point - long adventuring days are rarer than short adventuring days.
So now we have a hypothetical group where:
1) The DM refuses to put the PCs on the clock
2) The DM allows 'free of charge' Long rest recharges at the start of every session, regardless of if the PCs actually rested or not
3) The DM refuses to apply any of the rest variants from the DMG
4) The DM ignores the 'adventuring day' XP guidelines from the DMG
That DM cant exactly complain when things go a little out of whack. The DM is ignoring the DMG guidelines, refusing to police the Adventuring day or resource replenishment (in a
Resource management game), and is even making
active decisions to allow free Long Rest's at the start of every session,
despite that not being an actual rule of the game.
You keep saying this is the 'norm'. What evidence do you have to back this claim up?