That was really my point. In Classic Traveller the referee doesn't need to do these things.The GM still needs to plot out the campaign and individual scenarios
I started my campaign with a few NPCs and worlds rolled up. I rolled up the starting world after the players had rolled their PCs. We worked together to come up with some backstory that explained - in light of their "lifepaths" revealed by the PC gen process - what they were doing on the world. One of the players suggested that the world itself was a gas giant moon.
I then rolled a random patron on the patron encounter table, and connected her to a couple of PC backstories. And we went from there. (Eg I established some more backstory for the mission she gave them when one of the PCs - the ex-diplomat - seduced her (good reaction roll modified by Liasion) and then succeeded on an Interrogation roll.)
Classic Traveller supports this sort of thing well because it has so many systems for content generation, and has a lot of default elements that suggest interesting situations (starships, strange worlds, etc).
My own view is that more important than this - which reminds me very much of PbtA - is robust conflict resolution. That's not to say anything against the PbtA approach, but the reason I rate conflict reolution as more important is because this is what creates the impetus to action - situations arise and are resolved one way or the other, with new situations emerging out of them; there's not the "stalling at the locked door" that you rightly identify as a possible roadblock. And Classic Traveller, at least, supports "fail forward" pretty well and even has it built into some of its resolution frameworks, like working with vacc suits and travelling between worlds.games that don't give the GM levers make it harder. By this, I mean games that present pass/fail checks without grades of success or failure (or both at the same time) make following the fiction a tad harder
(As I've posted more than once in the past couple of years, the exception to this in CT is the syste for onworld exploration. It has some reasonable internal components, like rules for vehicle breakdowns and repairs, and for animal encounters and natural phenomena. But it has no overarching system for resolution beyond we get there when the referee says we get there. It's not a coincidence that my current campaign has featured only one episode of onwolrd epxloration, namely, the one where I realised that the system was weak.)
5e D&D has far more mechanical moving parts that CT, and so while your explanation of how it can be done "no myth" and "fail forward" seems compelling to me, I can see how some D&D players/GMs might find the prospect challenging. That's why I especially called out Traveller from the list of games in @McGibster's earlier post.
It's a rare GM who can consistently run decent games without preparing for them.
It was no secret to my players that I was rolling up a starting world, rolling a patron, etc. We were all there talking through the process.It's generally obvious to me when the GM hasn't made any preparations for the game
When I've run Cthulhu Dark we've likewise played no prep, no myth. The first time I had reviewed an old CoC scenario, The Vanishing Conjuror, but it was pretty weak and I think the scenario we came up with (involving a freighter carrying a shoggoth in its hold from Scotland to Boston for reasons that never emerged in play, and then trying to take it on to Newfoundland but sinking after being driven onto rocks by a tug under the command of a PC) was just as compelling. And required no prep other than me printing out a copy of the Cthulhu Dark rules (two sides of A4) before we played.
Our Prince Valiant campaign started with no prep - once the players had generated PCs, I chose what looked like an easy starting scenario from the Episodes Book. Since then it has never involved prep beyond me reviewing some Episodes to work out which would be fun to run.
Although different people take different approaches to this, as a GM I do like to know the rules of the system - especially if I'm the one introducing it to the group (as was the case for Prince Valiant, Classic Traveller, and our Dying Earth oneshot). But that's different from a constant burden of prep.
I tend to agree with @Ovinomancer that running without story prep is not as hard as is often suggested. To a significant extent it's attitudinal - about being comfortable relinquishing control over the direction of the fiction.