Yes but the temptation to steer failure outcomes towards your own predisposed interests as DM is just as likely as a pre-authoring DM steering outcomes towards the creations he wants to explore... in other words how can the later be a concern when pre-authoring but in improv there is no concern around the DM steering the direction of the "story" being created towards what he is most interested in improv'ing around?
I don't really see this as a helpful response.
It's absolutely steered towards a DMs predisposed interests. They can be very broad interests, but a DM can only present the options that are in their head. That doesn't mean that it will necessarily steer the story in a certain direction, but it will have an impact. Is that bad? Well, it's pretty much a given. The DM will run a game that caters in part to their own interests. How much is a variable, and whether that bothers the players is another variable.
In a prepped adventure, the temptation may be to keep the party within those predisposed interests. In a non-prepped scenario, there may not be a predetermined 'end-game' or even direction. And in theory, the actions of the players/characters can/will have an impact on the story and direction that is taken. But in practice I highly doubt that every DM is willing and capable to allow 'anything' to happen. The DM will have an influence on the direction of the story, even if they are determining the majority of things via random tables.
But if people didn't want the DM to have an impact on the story, they wouldn't play the game with a DM. There are enough resources to run a campaign entirely with random encounters and no DM. They aren't very enjoyable, even those that have a set story line like the Catacombs books did, because the input of the DM is crucial to an interesting game.
Not all DMs that prep railroad, just like not all that don't prep don't.
The tools are the tools. Just-in-time DMing is one of those tools. Prepping, which includes the use of published materials even as simple as a Monster Manual, are tools as well. The ideal to me seems to be that prep is focused on people, places, monsters, things, NPC motivations, plots and schemes, and such and a pool of available resources, and that the game play itself be one of more of a just-in-time approach. How much is dependent upon the individual DM and their skill at improv in the moment. Part of the purpose of good prep is also to help out when the DM is having an off night.
There will be a blend. I know that I don't usually predetermine where a specific treasure will be found, nor which monster, if any, will be guarding it. The players are in a dungeon right now, and the map, and certain fixtures, including some monsters are predetermined in their location, although for me that usually means undead, constructs, or bound guardians in a specific location. If they are intelligent creatures, the overall number and type might be known, but their exact location at a given point in time is not. The creature in the crypt is known, and the traps, puzzles, and such are often predetermined as well. The state of a given trap (has it been triggered), possibly not.
I might have pulled or randomly rolled some magic items that will potentially be found in the near future, but I don't know where. When possible I prefer for them to be in the hands of a creature that will use them. But I might not assign them until I randomly roll a given encounter. It also doesn't mean that I won't go to the sourcebooks and pull something else out.
But it is all shaped by my predisposed interests in that the stuff that's happening in the world around them is what I decided would be happening. If, how and when the PCs interact with these elements is a variable, much like a wandering monster. Sometimes I will roll randomly, sometimes the situation will seem like a good opportunity and I'll decide that it occurs there.
The results of those events might also be determined randomly or by me. Some as a result of character action, some not.
When I stop looking at the extreme arguments, and start looking at the tools and techniques themselves, I realize I've been doing a lot of this for years, just not acknowledging it as a specific approach. Getting a better feel for them, and utilizing them more efficiently is a good thing.
Ironically, how much I direct the actual story also varies from time-to-time, and usually has to do with being unprepared. Not in the sense of preparing specific locales, events and such, but having a pool of resources, which includes my own head, being ready when needed when following the story and where the PCs decide to go with it. I really like going deep with the research and prep. It's fun. But I can do that in areas where it makes the most sense, and will be applicable in many possible scenarios. Colorful NPCs, interesting plots and schemes and ways that things tie together in the world, waiting to be discovered, or not. Detailing a dungeon room by room, outside of basic descriptions and a map, or planning out the expected course of the night's game is not. Without control of the players, it may be a lot of effort for something that doesn't benefit the session at all.
The specific situation is important as well. If they are in a dungeon, then you can get more into the nitty-gritty of the next several areas they might explore. Their options are limited by the nature of the dungeon, although they don't have to follow any particular path, the number of paths to consider is limited. But most of the time, the best approach seems to be prepared for anything, by having NPCs, monsters, treasures, and potential plot points (rumors and motivations) available.
Ilbranteloth