WotC tried that in 5e. It sounds like few people agreed with what WotC told them.
Well, the problem is more that they agreed and didn't care? Or did care, but hated the consequences of doing the thing they knew was expected?
Because that's really the problem. 5.0 (and, I have no doubt, 5.5e) was designed with the mathematical expectation of 6-8
actually combat encounters per day, with somewhere between 3 and 6 rounds per combat, and somewhere between 1 and 3 encounters per short rest. If you go much beyond 8 combat encounters while keeping the short rests proportional, daily-resource classes begin to fall behind. If you fall below about 5 combat encounters, unless you raise the proportion of short rests (closer to 1:1 rather than the roughly 2:1 expected), LR-based classes clearly have far too many resources and will thus be encouraged to blow through as many powerful spells as they can, which substantially skews the numbers in their favor.
But.....actually
running 6+ combat encounters between long rests is exhausting for most groups. From my mostly-anecdotal but slightly-data-supported experience, the typical preference is 3-5 encounters between long rests, leaning toward the high end (so an average of around 4.3-4.5 per LR), and one SR every other encounter but occasionally after every encounter. My current (mostly-)5.0 group generally takes slightly more short rests, but that's because our Barbarian is the only one who has daily resources at this point. And if we did move to 5.5e, none of us would (Open Hand Monk/Barbarian, Wolf Totem Barbarian, BM Fighter/Rogue, and my PT7 Celestial Warlock), because Rage became semi-SR-based in 5.5e, as you regain one Rage per SR.
This is something Mr. Crawford openly spoke about in the lead-up to the "One D&D" playtest. Warlocks were explicitly described as falling behind because they weren't getting enough short rests per long rest, because players were fighting fewer combats per long rest than intended, etc. As a result, Warlocks genuinely lacked for the resources to keep up with Wizards or Land Druids or Clerics who were pumping out spells left and right--sure, the Warlock gets all their (5th-or-lower) spells automatically upcast, which
is very nice, but if you're only getting one SR per LR, that's four best-level slots...vs the 3-4 that the Wizard can get via Arcane Recovery. That is, for character levels 3, 5, 8, and 10, the Wizard gets three max-level spells per day plus all their lower-level spells; for character levels 4 and 6, it's four; level 7 and 9, I admit, it's only two. But that still means that at nearly all pre-11th levels,
if there's only one SR, the Warlock is casting at best two more max-level spells than the Wizard, and oftentimes exactly as many, while the Wizard still has 4-10 lower-level slots to draw on as well.
If--and I want to stress that this IS an if, because other preview content has been pretty positive IMO, so I'm trying to be generous here--
if their advice really genuinely does boil down to pretending that the adventuring day just doesn't exist, that will be a major disappointment for me and will cast a pretty serious pall over this portion of the DMG. I'm very much hoping that this article is dramatically overplaying things, and that the adventuring day still matters, it just might go by a new name or be presented in a more ground-up fashion (e.g. guidance on how to manage many different proportions of encounters per LR, encounters per SR, and SRs per LR.) If the latter is closer to the true situation, then I might still have some beef with their presentation, but the core information would still be good, constructive advice.
You make it sound like monsters won't have Challenge Ratings anymore.
Oh they will! They just almost certainly will be about as useless as they were before, little better (and sometimes much worse) than eyeballing everything. But if they're genuinely trying to pretend that adventuring-day resources
don't exist, those CRs are liable to be even less productive than they were in 5.0.
Or that new DMs are going to say, "oh, man. I just can't challenge my PCs. But when I do, they all die. Oh well, I'll just keep running monster encounters purely by die rolls, stat blocks, tables, and Matt Mercer advice. Fie to Rule Zero!"
I've no idea what Rule Zero has to do with this, other than a specious "because you can rewrite the rules, the rules can't actually cause problems!" argument. But DMs have been saying
exactly that about 5.0 for its entire decade-long run (really, decade-plus, since there were rumblings about this even during the playtest!) Being very charitable, I suppose I could read this as "new DMs are going to
continue saying..." but that's a pretty weak argument, because the expectation of continuity with the past unless clearly overridden is hardly weird. That's what "backwards compatible' is usually understood to mean.
Oh, wait. That does sound like WotC's latest mantra...
I'm afraid I don't get the link between the reference ("it puts the lotion on its skin", yes?) and the WotC mantra.