I think this gets right to the heart of the matter, and is correct: it is not possible to really provide both experiences in the same ruleset, at least not and fo both tight and challenging.
tight rules do not mean things have to be challenging, it just means they are pretty predictable
5E works fine for telling less combat heavy stories, as long as nobody at the table cares about a tight tactical challenge
6-8 encounters per day does not feel like a combat-light story. If people can ignore that, they can also ignore 3-4 better controlled encounters and either reduce the number or challenge as needed.
I see no upside to loose math, there is nothing you gain from it. Anything you can do with loose math, you can do with tight math, there are however things you can do with tight math that just cannot be done with a loose one. The only ‘question’ is whether tight math is worth the effort, and WotC apparently has an answer.
Ultimately I don’t think it is about tight math for me anyway, but about tradeoffs. If resting is always the strategically best way to go about things, it gets boring.
No idea how tight DS!’s math is, but I like its approach where the characters get stronger from not resting while their HP deplete. This is interesting whether the math is tight or not. It offers a dilemma and a choice. D&D’s approach does not, it is up to each DM to turn it into one (potentially against the interest of their players) or just roll with the punches / frequent rests.
(see again, Critical Role: nearly every fight is a foregone conclusion cakewalk but the players haven't figured that out in a decade of playing).
they are paid actors, their job is to make things exciting to watch. Pretty sure they do know that most fights are cakewalks, they also know that presentation matters if they want a large audience