To me it mostly has to do with the fact that the groups I've played with want to get to the end of the "mission" whatever that may be(stop the cultists from destroying the world, save the mayors daughter, find the magic artifact, etc).
They will continue to seek this goal for a short while before they will give up due to being underpowered.
Assuming the formula in the DMG works the way it is supposed to, each encounter of CR=the average party level should use up 25% of the resources of the party(It never works out exactly that way, but lets assume it's true). What I've found is that a party is willing to go into a fight with maybe 50% of their resources, but any less than that and they wisely head back to rest.
If they fight a hard battle right away, they'll rest right after it. If they fight a lot of easy encounters in a row, they'll turn back after 5 or 6 encounters. The idea of the new system is that 80% of their resources is still enough that they aren't afraid to continue.
Sure, they aren't full, but do they want to risk the enemies raising their dead or hiring and summoning new guards while they are resting? Do they want to risk the fact that the kidnapped woman might be killed tonight? The answer, in most cases, is no. They'll push on with 80% resources.
On the other hand, I've seen groups not care whatsoever about the plot at all as soon as there is a serious risk of their own deaths: "I know that the evil cult kidnapped the woman and we heard they were going to sacrifice her. But we don't know WHEN they are going to sacrifice her, right? So it COULD be next week just as easily as today, right? And, we're all out of ALL our cure spells and the wizard has only 2 spells left. Frankly, I don't think we'll survive anyways. Let's rest for the night and hope she isn't dead when we get there."
My players don't WANT to metagame...but they'll do it when the system forces them to. They'd much rather their character be the heroes and press on, even in the face of danger...it's just that at a certain point the danger becomes so great they RELY on metagaming to get them through("The DM won't have them kill her tonight, since he put us up against so many hard encounters in a row, he must have expected us to rest the night before saving her").
I see nothing wrong with each encounter being one that could go either way and the PCs having the ability to fight 20 of them in the same day.