Why can't it more consistently be "fail backward", though?
When breaking into a kitchen and failing the roll, why can't the narration be that a previously undetected alarm goes off, or that an unexpected electrical trap in the lock shocks the thief for [a bunch of damage, however the system does such things], etc.*, while - honouring the root 'fail' roll - the door remains locked? One of my favourites for such things is that the thief, thinking she was unlocking the door, has in fact just locked it i.e. for some reason it wasn't locked to begin with.
Because this doesn't move the game along or provide an interesting consequence. It's merely punitive. Or worse, if the thief had checked for traps and rolled well--yes, there are traps the thief might not be able to see, but you're making one up here on the spot for no reason
other than to "fail backwards."
(The last one is, IMO, pretty dumb. You're assuming that part of the thief's door-checking routine doesn't involve checking to see if the door is opened. If you did that to me, I'd start narrating every time I inhale, just in case you decided I was holding my breath. I'd make a little recording and have it play on a loop.)
* - or even the old standby "the thief breaks her tools in the lock", but that one gets overused and thus a bit stale.
Lockpicks are very thin and fragile; having them break often is fairly logical, unless they're made of, like, mithril.
Ah, there's another place where I differ in philosophy from this lot: even the most competent people aren't perfect, and every now and then when under pressure they'll mess up on something they otherwise do consistently well all the time. The thief might pick that lock 100 times out of 100 while practicing in the guild hall, but the stress of doing it in the field when it counts - even if she has all night - might cause her to mess it up. And from all external appearances, that'll look like incompetence every time.
As for "no whiffing": even the very best hitters in baseball whiff on a fairly regular basis. Why should PCs be any different?
Fail forward takes this into effect. Here's something I found on an
old reddit post, with the idea that the PC is chasing a mysterious assassin:
a. You lunge at the assassin, ripping off his cloak. He shrugs it off and makes good his escape, but now you have an article of his clothing and possibly a clue to his identity. (The player gets a new challenge/storyline)
b. The assassin turns around and throws a dagger at you. It is poisoned. Someone will need to identify the poison in order to cure it- a possible clue (A new challenge and the player gets more than she bargained for)
c. You lose track of the assassin in a dark alleyway. Suddenly, he lunges at you out of the shadows (The player gets put at a disadvantage, but gets another chance at her goal)
d. You grab the assassin and wound them. He struggles with you, quickly escaping your grasp. he scurries up a wall and looks down at you, bleeding. You get the sense he is memorizing your face. (The player/party now has a new antagonist, the nameless assassin is now a character.)
Note that none of these involve the PC
automatically catching the assassin. What they all do is make the adventure continue to move.
When it comes to picking the lock, if there's no consequences for failure other than "nothing happens"--there's no deadline they have to beat, no monsters or guards that might find them if they take too long, nobody relying on them, nothing to learn from the lock itself, a failure simply means "nothing happens"--then why have them roll at all? Why not just say that the thief picks the lock? Having them roll, whether it's a failure or a success, is just a waste of time.