What? Im sorry, i literally dont get what youre saying here?
What he's saying relates to what you wrote later. Many players don't want to deal with your concept of the "adventuring day." Those rules are considered optional, not a regular part of 5E. They view an adventuring day and control of that adventuring day using their agency as important. When you as a DM modify it using optional rules you need to let them know, so they can choose to accept your modifications or not. I would not accept your modifications. I would consider them limiting my agency. I get to use my abilities to control the game flow, not just you as a DM.
That's why if you take up my offer, it will be the 5E rules without optional rules. If you allow feats and magic items, we can negotiate what optional rules I will allow you as a DM. This idea that the DM can just tell his players, "Screw you. I'm using what optional rules I want" is not something I allow as a player. I just don't play with people that want to do that.
Its not 6-8 encounters per day. Its 6-8 per adventuring day. So in other words, it could be 6-8 encounters in a week (again the rules for longer adventuring days are right there in the book, along with other optional stuff like feats, multiclassing, and magic items).
Its really not that hard (as DM) to consider and place a time limiting event/ time constraint on your adventures in a non intrusive way to enforce the 6-8 paradigm for around 50 percent of your adventures.
This is not how the base 5E game runs. Sorry, the long adventuring day resting rules are optional. If your players don't want to play in games that use them, you don't get to use them. Or you find other others players and they find another DM.
If you dont have the skill to place such time limits in adventures in unobtrusive ways, the DMG provides for the longer rest option.
Skill is irrelevant. I'm a player. I dictate to you how my abilities will be used, you don't dictate to me how they will be used. My character's life is in my hands. If I don't feel like buying what you're selling as a DM, my PC leaves. What do you do as a DM if I simply say "No. I don't care about this and your artificial limits. I go find something else to do."
If resting equates to an auto fail for the adventure, or imparts a complication to the adventure that outweighs the advantages of resting, or reduces the award in excess of the benefits of resting, then it is irrelevant how easy it is to rest.
I find something else more profitable and less limiting if a DM puts me in this situation.
Magnificent mansions, rope tricks etc simply remove one type of resting limitation from the game (hostile environment). They dont help time limiting factors.
I have to make a character that cares about time limiting factors in all your adventures? My PCs make mostly neutral or less savory characters. Their goal is survival and their own power. Time limiting factors equal higher chance of death and less chance of profit and less agency by the PC, which means avoid such limitations and find other things to do.
By all means allow PCs with access to such tricks the occasional chance to use and benefit from them. But by the same token bear them into your preparation when designing the adventure in the first place. No amount of Leomunds tiny hut and Rope trick is going to help when you need to save the princess by midnight or the demon gets summoned, or the BBEG is now at risk of hearing about your party and it gives him equal time to reinforce (or simply relocate resulting in an auto fail) or if your employer needs the macguffin by a certain time or the party doesnt get paid.
What if the PCs don't care if the Princess is saved at midnight because they're more concerned if there are magic swords in the castle? Do you change the save the princess scenario to "This magic sword only shows up at midnight once every thousand years and you must fight your way to obtain it?" If you do, cool. That would be the more likely way to motivate my PCs. You had better make it one great sword or they won't fall for that trick again. If the demon gets summoned, what would you do if the PCs do the following: I don't think we can make it before the demon is summoned, let's get the hell out of here. Then they leave allowing the area to be destroyed.
Seriously, player agency is a big deal. That means controlling the world, not letting the world control you. My players including myself work very hard at this because the way to survive and increase in power (level) is to not let the DM dictate terms to you. That is the very essence of player agency. If a DM attempts to disrupt that too often, he will make players unhappy. Thus trust must be built that when the DM uses unobtrusive ways to stretch the parties resources, he does so in a fashion that will be fun and give them a very high chance of success. Killing PCs too often leads to players leaving the game and not wanting to play with you. Which is understandable given that players put their all into character design, they don't want to see their creation destroyed because a DM is using optional rules or putting them in situations where death is a high possibility due to time constraints they don't care about.
A lot of what you consider "skill", I look at more as a removal of player agency and expectation that everyone is playing someone heroic that will care about your time constraints. In my experience, players care more about their character's survival. They won't toss away characters they've spent a long time leveling because a DM feels like pressing the 6-8 encounter paradigm. Which is probably why you and I see things very differently. Maybe you've never had players look at your scenario and flat out say, "Don't care. I leave." The entire group leaves to find something else to do because they feel like you're putting them in a bad situation. I've had this happen and I've done it myself, even if a few party members have died.
That's not to say my group hasn't played their share of heroes willing to lay down their lives. That's not the majority of parties, especially now that we've been playing so long and are jaded. Now we like our characters to survive. We don't fall for DM tricks, so they make mostly neutrals so they can't be manipulated one way or the other by DMs looking to have them "save princesses by midnight." Their aim is to survive and gain power in that order because goal two doesn't work if you don't meet goal one.
Kudos to you if your group isn't that jaded yet. I can't say the same. Even in
Out of the Abyss which I'm running right now, out of six characters four our neutral including the paladin (yay for neutral paladins), one is good (poor bastard is the only pushing for good activities), and one less than good (he could give a rip about time constraints).