Fundamentally, 'Monty Haul' (at the time it was originally used. Nowadays it can mean 5 different things to 4 different people) was a reference to people playing an existing system in ways someone with a platform (Gary, who developed the thing, which may or may not be relevant) thought out of sync with best practices. Declaring a whole system to be Monty Haul seems outside the intended use of the term. For the time, I can understand the point. As DEFCON 1 points out, that was when XP approximately equaled GP, so a lot of it was people levelling faster than Gary thought prudent*. And there's merit to that -- there wasn't a lot of stuff to get after name level for most classes, and the high-level spells were better suited for big-bad enemies the PCs should be defeating than really for PC usage (at least until some more high-level play structures like plane hopping madness were also developed). Opining that getting to that point was intended to take several years instead of months or the like was not an unreasonable position (TSR didn't really go about it in a very level-headed or diplomatic manner, but that's another issue).
*it could also be magic items, but honestly the oD&D and AD&D treasure tables gave out quite a bit of magic item loot, and stories abound of characters in his oD&D game being awash in wishes, so this would be more of a hypocritical focus if it were the case.
Regarding 5e -- IMO, it's not 'Monty Haul' (again, I don't know how that works in the original context of the term). I would say that its' defining quality is 'default to an easy setting' -- I won't even say it defaults to easy mode, since if people find it too easy they can just keep taking on greater threats until they are in over their head (and it becomes hard again), but I think we can safely call the default play rules an easy setting.
Fundamentally, I think this is a great move that TSR should have done very early in the game's evolution -- at least for B, BX, and BECMI which were billed as being for ages 8 or 10 and up* (but were mostly oD&D rewritten to a 4th grade reading level). I can't begin to count the number of friends I had in grade school who seemed the target audience for the game but who tried it for a day or a summer but didn't stick around, oftentimes because they died for the 18th time before 3rd level and decided the game just wasn't fun.
*or some specifics, away from books atm.
Of course, then you need guidelines for what to do when you want to move the challenge up*. 5e has some relatively clear and straightforward options in the DMG (as alternates, and also a roadmap for adjustments of your own), but then huge swaths of gamers look right past them and continue to complain about the game being too easy (and I can't really blame them). Whatever 'right way' there is for setting up optional difficulty moderation, they clearly botched it for many-to-most.
*preferably other than just going after higher and higher challenges within the existing system, which IMO ends up becoming really swingy in a 'everything is fine until everything is a complete disaster' kind of way.
That's not how people play because gameplay is limited by the constraints of how long those encounters take & how long a session lasts. Sure you can spread that out over multiple sessions but the rest mechanics are structured to be easy for players to force through no matter what the GM throws out shy of outright Fiat & the party is still able to trivially blaze through all of the prior filler encounters with ease up until the last fight or two. When the default assumptions fail to account for realities of things like table & session time in any way shy of throwing it to the GM to solve those default assumptions are a failure of design.
What I don't really get is this: Once the game left the Sandbox/West Marches dungeoncrawling campaign style (that may or may not
ever been standard play for most groups),
all versions of D&D* have that. Barring DM imposed time clocks, players could always go out and rest overnight, two nights, or a couple nights (if your cleric couldn't cover all the HP loss in a single memorization cycle**, and in AD&D once 10-minutes/spell level starts adding up) and come back with a full refresh of abilities. I agree that it is a fundamental issue that the game never really solved to satisfaction (other than perhaps admonishments against, similar to the call of a playstyle Monty Haul), but I don't really know why 5e gets special mention on this. Maybe because there number of save-or-die effects have also been ameliorated (removing one of the consequences other than TPK and slow wear-down of resources), or because HP return overnight rather than merely 'usually over two days (if you also want full cleric spells).' Both were changed for reasons I understand, but I can also see some unintended consequences in the aftermath.
*barring 4e, and even it has daily powers.
**and if the only cleric went down, well then I guess then things became a challenge
In my case "Monty Haul" isn't just gold and magic items. It's getting any kind of reward for minimal effort. This can be XP, Levels, Titles, Story Developments, or - heck - even the reward of playing the game itself. Most of the time my players can pass through on auto-pilot, halfway paying attention to the plot, combats, etc. They're still rewarded with the game continuing on as if they had played masterfully, using every resource, been thoroughly engaged, etc.
I'm unsure how the system could be doing that. How does the game system stop story developments from being challenging? How does the game continue or pass them through even if they are on autopilot? The only thing the game is doing is making the combat (and dungeon-crawling) aspects of the game relatively easy.
Sure, if rescuing the merchant's son is too easy against the 8-member band of brigands, well sure the DM might have to put them up against 12 or 16 brigands if they want it to be a challenge. But the party still has to negotiate reward with the merchant, find the brigands, scout their camp, figure out how to rescue the son (and not have him be killed in the resulting scuffle), and all the other things that happen in any game system.
I'm to the point (especially with the 7th level group) where there is nothing that can be done in the constraints of the 5e ruleset that can challenge them. Throw enough enemies in to challenge them, it becomes a grotesque slog of 100+ monsters that would take weeks to run. Throw in the rare enemy that can challenge them, and it's a TPK. Let there be ramifications in the plot for a story failure, and the campaign world is destroyed (not that they really care anyway).
So you think you can't let them not win because the game world is destroyed? I don't know how the game system can address that.
For me, a satisfying combat should have numerous times where several characters feel on the verge of death. If combats don't feel challenging, why bother? Especially when the party calls all the shots about when they heal, recover all spell slots, etc., the only thing you can do is threaten their characters and chance of success in the campaign.
The first part is going to be about preferences. I know several people who disliked 3e specifically because it seemed to want everyone to be extremely powerful but always feeling like they were a single unexpected event away from complete annihilation. They would prefer the 5e model, and I'm guessing you preferred 3e.
To the second, fundamentally, if a DM feels they can't call the shots on when the party can recharge, there's nothing else to say (but again I don't know how this is really different from all D&Ds). Maybe in your case, 5e (and most all other D&Ds) as your group seems wont to play it (I'm assuming pushing back against you trying to control how frequently they can rest) is Monty Haul in the original intent of a playstyle you find at odds with how you see the game best going.
