That's true. Only when the outcome is in doubt and failure has meaning should there be a roll.
Well, you'll want to talk to
@Lanefan about that, as he's rejected
both the idea that failure needs to have any meaning,
and the idea that success needs to have any meaning (I presume separately, as if both are true simultaneously I'm pretty sure even he would say "don't bother", but I could be mistaken.)
This, for instance, is one of the reasons why I don't think it so horrendous to talk about, for example, a guard or servant* to walk past a secondary servants' entrance into the house. Keep in mind, in medieval, renaissance, and early modern periods, servants had their own halls, staircases, etc. they were supposed to use. At essentially all times other than "direct personal service" situations, servants were supposed to stay completely out of sight. Dusting, sweeping, mopping, etc. were to be done while the family was asleep, unless a sudden emergency occurred (e.g. a major spill caused by the family or guests), or the family needed special service (e.g. "we're hosting prestigious guests for supper, get the house
completely spic-and-span!") So it's very much A Thing that, during the wee hours, various servants would clean, prepare, organize, etc., etc. so that the family could rise fresh and ready the next morning, seeing only their direct personal servants (e.g. the ladies' personal maids, the gentlemen's manservants, the butler of the house, etc.)
Most locks, so long as they aren't some bizarro newfangled design, are not
that much of a challenge to pick, having watched a fair amount of the Lock-Picking Lawyer--assuming you have the right tools. Hence, for most locks, what matters is less
can you pick it, because the answer is most probably "yes"--what matters is how much time it'll cost you. A robust lock might take a good while, especially to one unfamiliar with its design, but unless it genuinely requires a specialized tool, whether or not it
can be picked really shouldn't be in doubt. (And, I'll note, if it
did require a specialized tool to pick it, that would imply the lock couldn't ever have been picked in the first place, which has been explicitly ruled out--success
was possible when our Thief began picking.)
As a result, a difficult-but-not-impossible pick might have to be stopped, not because the picker cannot do it whatsoever, but because doing so is no longer safe. Most security measures work exactly like this. Security isn't, and cannot be, absolute. Instead, security measures are an effort to make it too risky to try. A lock that the best lockpickers can only get through after an hour's concerted work is a very good lock, because an hour is an extremely long amount of time for the would-be burglar to just
sit there, doing nothing, focused intently on the lock. That's a risk almost all burglars would avoid--all but a very few, very brave and usually very foolish, would avoid it.
When coupled with the above, is it really so unreasonable to say that the result of "failure" is "you weren't able to get through the lock before attention was drawn to the door you were trying to get through"? That seems like quite a reasonable, and indeed
realistic, answer. A lock that is simply impossible for a trained (but perhaps green) lockpicker to pick, solely due to skill level, is
less realistic than one that is simply very challenging to pick, and which would take too much time for this trained(-but-maybe-green) Thief.
The fact that this advances the fiction toward some kind of conclusion (in this case, toward discovery and thus either failure of the stealth mission, or needing to risk violence to complete their objectives) is thus a bonus on top of an already-realistic situation: Servants
are milling about even though the family is asleep (a typical situation in manor-houses for centuries in Europe, from medieval to industrial times), guards
are patrolling, and the Thief is much more likely to be skilled enough to pick the lock
eventually but not soon enough, rather than being simply incapable of it at all.
*"Cook" was the specific term used earlier, but it could be
any servant, cooks are just likely to be near to one of the most plausible servant-entrance doors of a manor house.