"Stubborn is better than Fearless!"

No, it isn't.

Let's get this straight: the No Retreat rule is punishing, yes. Especially in multiple combats its effects are rather ridiculous. Magnifying this is the fact that ATSKNF makes a large percentage of the armies in the game immune to being swept from losing combat, which is nominally the thing that makes combat resolution so punishing. It still is reasonably dangerous to anything but Terminators, but not nearly to the degree that No Retreat on low-save models is.

That still doesn't make Fearless worse than Stubborn.

Outside of combat, Fearless is purely better; there's simply no argument. Fearless units never fall back at inconvenient times due to Tank Shock. Fearless units never get Pinned. Fearless units don't care about Fear the Darkness or Weaken Resolve or any of those other things. They do exactly what you tell them to and they keep doing it until they're dead.

40K is a game of dice; all of us are dependent on luck to some degree. The more you can reduce this dependency, the better. Fearless units do this wonderfully, because even Ld10 fails 9% of the time, and Ld9 17%. You do not have absolute assurance of non-Fearless units doing what you want; good confidence, but sometimes your Terminators will break and run from the position you had them in and your strategy will come apart. Switching from Tyranids to BA has made this abundantly clear to me- you MUST be prepared to occasionally lose one of your unit's turns due to a poor roll if you aren't Fearless. Stubborn doesn't do that for you- in fact, Stubborn doesn't really do anything for you outside of combat.

Of course, in combat is the big sticking point for most people. As mentioned earlier, No Retreat can be pretty punishing, but that's only part of the truth: for non-Marines, all combat resolution is pretty punishing. Would you rather your Orks were Ld9 Stubborn instead of Ld10 Fearless? Would you really? If you have, say, 20 Boyz left in the squad, losing five or eight of them to combat resolution is painful, and it's guaranteed- but losing the entire squad to rolling a 10 on the dice is also very harsh. By many measures, it's a lot worse- the Boyz are basically just meat shields for the Klaw in most cases, so it's only losing the last one that matters. With No Retreat, you keep the Klaw alive until the very last second (obviously), allowing it to inflict maximum punishment, whereas a Stubborn unit as above stands a good chance of being prematurely cut. It all comes back around to the dice- you can count on Fearless Boyz sticking around to get hits in each turn, but a Stubborn mob in combat might just go away at any time.

Looking also specifically at Imperial Guard, your Ld9 Stubborn is more reliable thanks to the reroll... for a price. For 20pts per reroll, in most cases, because those sarges are usually carrying a Power Sword as part of the overall blob strategy and thus are not just disposable models. Every time that 17% chance crops up, your offensive power decreases by a significant chunk. I would be much more inclined to just throw away a couple extra troopers for No Retreat instead, were that an option.

I'm not saying Stubborn is a bad rule; for tarpit units like Guard blobs, it's pretty much perfect. And, in some circumstances, it is better than Fearless- but most of the time it is not, so stop pretending that it is.