Hard disagree. Without the timer, the challenge in M+ would be dealing with horrendously unfun things, like AoE’s that OHK you, spells that will insta-gibb your healer if not kicked, or bosses having their HP/damage boosted, they become an almost insurpassable wall.
Ever tried to do a boss like Cragmaw or Yazma on a high Tyrannical week? That’d be what EVERY boss would be like. Meanwhile, every trash pack would have to be like the center pack in Atal (with the two big Dinos), because otherwise Trash wouldn’t matter and the meta would shift to be purely boss focused.
Having a way to “fail” a key is the only way to prevent the keys from being designed in such a way that bashing your head against them for 2 hours doesn’t become a viable strategy.
The “RPG” part of that was ruined long before M+.
Those dungeons you describe haven’t existed since Vanilla. And even then, they were very much few and far between, with the only dungeons that even vaguely meet that criteria being BRD and Mara (WC too, if we’re being somewhat generous). Everything other dungeon in Vanilla (and since then), has been more or less a straight run to the end, with maybe a branching path and optional boss here and there.
Removing the timer isn’t going to suddenly make Blizzard change how they develop dungeons, when they clearly do not like designing extremely long, non-linear dungeons. If it did, we would have seen those kind of dungeons in BC, Wrath, Cata, MoP or WoD.
If you want a D&D style RPG, I’d recommend Divinity or Pillars of Eternity, not WoW.
Not really. All that would encourage is waiting for Lust and full cooldowns for every single pack, and the challenge being designed around that.