Meant to be played, as in, according to how WotC describes it in the DMG and clearly balanced the mechanics around.
What you seem to find confusing is me, describing how I feel the DMG guidelines are impacting the game, that I used to run in a different way.
If I understand your approach correctly, you say, that I should regularly push the party to its limits in terms of resource management, which would be equal to running the recommended amount of encounters per long rest. That way the players will still conserve their resources when there are days with less encounters.
The thing that bothers me about that is, that a level of arbitrariness is needed to create the constant feeling that the party is in a "warzone", as you have described earlier.
Sometimes that just does not suit the scenario the players find themselves in. My players will infer that from the game-world.
Then they will use long rests more readily, especially if the time constraints of the normal long rest are barely a factor. And rightfully so.
If for you it is no problem to have stretches where encounters are merely something the party deals with easily, that is fine. I like them to be challenged in any scenario of play.
But instead of handwaving the logics of the scenario and essentially trying to balance the resting mechanic by adjusting my encounters, as stated in the OP, by either making them harder or throwing more "warzone" like constant attacks at them as you suggest, I just increase the cost of a long rest.
The cost is time and it is always valuable, because the game world changes.
That way I can have a scenario where the party could theoretically safely long rest at any time, but the party doesn't want to. As opposed to, them having to worry that some assassins are trying to murder them every night and that is why they conserve their resources.
I suppose if I experienced how you played before to now, I might have a better picture of it.
My dislike of the DMG guidelines is that it creates too much routine when I have seen people using it. Also since a long rest is normal 8 hours, the "adventuring day" is really the period between long rests, the idea being that after so many encounters, a party's resources will be low or depleted and further encounters create greater and greater risk.
Now, by "war zone" resource management, I mean that, while a single fireball could crush a group of kobolds, the threat they likely represent is not worth the resource. Sure, it is a power-trip to wipe out a threat with a single spell, and if the character is "going back to bed" and getting a long rest afterwards, why not, right? Well, while I don't expect assassins to come in every night while resting, I like a game where resources are used as
needed not just when desired because you never know what is to come.
With the DMG guidelines, a lot of that is lost IMO.
So, it isn't that you plan to push a party to its limits. It happens organically enough if you design/play with a mindset of real life drive, concerns, etc. The tipping point comes when the party realizes they are approaching a point of no return. As you understand, other times the encounters are easy and of little threat, but in the concept of a real world that makes sense.
I find it abhorrent that XGtE has encounters based on party level, not monster rarity. I mean, do random encounters and such become more dangerous simply because the characters are higher level? Is the world just becoming more and more dangerous? Did those threats not exist before?
At level 1, humanoids and such are the common threats in most games. At level 20, it is fiends and dragons. So, what happened to all the orcs? Where have all the goblins gone? And where were the fiends and dragons at level 1?
Let me ask you this: how often does your party seek to avoid conflict or need to run from a fight because otherwise they know they will die???
I use truly random encounters. In 2E, I had a game begin, very first encounter, was an age 8 green dragon. YIKES!, right? You bet. How was I going to work that into the story? How was it possible the characters would survive? Well, since we started in a small town, the character awoke that morning to the dragon raiding the town. They hid, they ran, some of their friends and family members were killed. After the dragon left, what could they do? There was no way
they could fight a dragon, it was insanely beyond them. So, they decided they needed to alert the local ruler and find heroes who could handle the dragon.
And how often do they encounter something that runs away from
them? At lower levels, not often, but imagine being in world of magic and you are an orc in a small war party. You happen about 5 travelers, but three are riding phantom steeds, one is on a pegasus, and another striding alongside a large wolf. Some have heavy armor, others rich robes and staffs, etc. Even outnumbering the party 3 to 1, the orcs should hesitate. Why? Because in a magic world they understand those are likely heroes with power. A single fireball could destroy half of them! Unless they could ambush them, get the upper hand, or something, the fight is not worth the risk. Most of the time, the orcs should (IMO) seek a non-hostile solution or simply run.
I understand a lot of this depends on the game world. But, if I was a caster, and your solution was to nerf my spellcasting, I would also be upset. The fact you have to find a solution to deal with a new problem indicates to me there might be a better solution to your original problem.
All-in-all, I am not trying to be an ass about it. If there are problems as you see it in your game, and there is a good solution that works for you, great. Sorry for the long response, I generally prefer to keep them brief but this is a topic of interest to me and seeing how other tables operate.