Can the door not be opened any other way than by picking the lock? Forcefully-applied boots? Repeated use of an axe or hammer? Removal of hinges? Knock spell? Shrink spell? Warp Wood spell (if the door has any wood to it)? Someone goes gaseous or shapeshifts into something really small or even Dimension Doors then goes through and opens it from the other side?
Simply concluding the players are cut off from part of the adventure because they can't pick a lock seems like very stilted inside-the-box thinking. And even the lowest-level parties have access to the non-magical possibilities noted above, plus others.
It depends on the context, but for some of these:
- Some doors can be forcibly opened that way, some can't. Many dungeon doors are made of stone or metal, for example--that's been my experience with doors in 5e, for example. Wooden ones are rarely an impediment, and stone ones aren't realistically gonna yield to mere hammer blows.
- "Removal of hinges" is one of those ideas that only makes sense if the GM isn't really thinking through the design of the dungeon. If the hinges are accessible from the side of the door that's meant to keep people out, the lock is worthless and the architects were idiots, which might happen very rarely but can't be relied upon.
- Most of the spells you've mentioned are not a reliable tool for several reasons. In older-style D&D play, such as your game, IIRC spells are only received randomly and there's no guarantee you'll find them out in the world either. I'm not familiar with warp wood (sounds like a Priest spell?), but that depends on the door being wood, so same issue as the first bullet point.
- Depending on the rules involved, it's not always possible to shapeshift into something small enough to slip under the door. Certainly in Dungeon World that'd be a thing, for example, but in 3.5e, Druids cannot shapeshift into any size smaller than Tiny, which is two size categories too big to fit under most doors (you need to go past Tiny and Diminuitive down to "Fine", which is <6 in tall/long).
- Dimension door is almost surely a success if the door can then be removed afterwards, but is a pretty powerful spell to blow on just potentially getting past a door (4th level in every edition I can find--surprisingly, even in 3e which often had spells at wildly different levels depending on class). Players might rightfully question the worth of such a thing.
If lots of other groups can't or won't think outside the box now and then, that ain't my problem.
Perhaps, perhaps not. Surely, though, it would be good for GMs-in-general to know and have the use of tools that can address these issues? Surely it would be beneficial to have ways to teach GMs without forcing them to make stupid mistake after stupid mistake after stupid mistake in order to finally stumble into wiser ways to approach a problem?
Like...that's literally what makes humans special. We can build up one lifetime's worth of knowledge...and then condense it into a few books, perhaps a small bookshelf worth of reading, which another human can then digest and understand in mere days, perhaps weeks. Developing tools and processes and procedures and guidelines and rules of thumb and (etc., etc., etc.) so that those who come after us
don't have to blindly stumble in the dark until they find the way a thousand other people silently found already but failed to communicate to them.
That's one of many reasons why I talk about tools of various kinds (procedures, processes, guidelines, rules of thumb, best practices, SOPs, etc.), and why I almost always ask about them when I want to know more about a style I don't yet know. Those things
are human power in action. And it's why I see TTRPG design (not play,
design) as a technology; because our techniques can in fact get better. We can recognize where there are flawed behaviors, like the whole "force the rogue to roll Sneak and Move Silently
every five seconds, and they are immediately seen and captured/attacked when they fail" problem, which is addressed by
Let It Ride, a technique that even many very, very experienced and otherwise high-quality GMs simply do not learn on their own.
We can--and should--identify places where problems commonly occur, or where there is a serious risk of very severe problems, and look for ways to prepare ourselves and those who will come after us against such pitfalls. That's not in any way a deprecation of the importance of human judgment. It's not in any way replacing humans with robots. It is simply recognizing that it is often easy to run into problems you don't realize are a problem, and even when you DO know there's a problem, knowing you have a problem and knowing how to solve it are VERY different things. Giving others--both contemporary and in posterity--tools to address the problems
we know we have means they can be focused on dealing with whatever
new problems will inevitably arise in the spaces between the up-front problems we dealt with.
Except that's not a fail state!!! Getting through the door is a - no, it's the - defined success state for this roll, and thus by RAW needs a 'success' result in order to occur in any way. A fail result of any kind means they do not get through.
It depends on whether the party HAS to get through the door or not, doesn't it? One of the reasons behind "fail forward" is specifically to teach GMs that there is a different way to deal with a "single point of failure" problem. That is, sometimes you're going to only realize something was a single point of failure too late to directly address it--or you're improvising and didn't think that far ahead, or you truly want this to be a single point of failure because that creates tension, or whatever else. But a single point of failure where the only result of failure is "the game grinds to a halt because nothing happens
nor can happen" is pretty blatantly a bad thing.
Fail forward, as a rule of thumb, means that even if the party gets struck by such a thing, the pace of the experience and the enjoyment of play don't get drained away as the party sits there, waiting for one of their schemes to finally,
finally, FINALLY open the stupid friggin' door.
