No flips for you!
But not mine, which is the point I'm making. I don't prep that objective -- the players do. I prep some challenges, and how the players choose to interact with those on the way to their decided goal is the game. Here's a reasonable example. My players' PCs are following up on their own agendas, but occasionally want to fulfill a contract (they advertise services as recovery specialists in Sigil), so I have a few generic preps lying around that I can brush off fairly quickly and plug in if they just want a session or two of loot and monsters. This time, it was recover a doodad from an abandoned dwarven smithy deep under a mountain in the Outlands. A pretty standard dungeon crawl, the details of which are rather uninteresting for this discussion. The point is, they failed at the end when facing the aboleth that I had set up as the final monster. Did I pick this? Yup. I prepped it because I found it interesting, but it was a side quest style thing -- no story outside of the side quest. Until, that is, they failed and fled, which allowed the aboleth to escape with the doodad. Now, they had a bit where they decided if they wanted a blemish on their record or not, and the players decided not. I had nothing to follow this at that time, so I ginned up a quick skill challenge to determine what they could find out about where this aboleth might go. I run my skill challenges a lot like how some other games work -- there's the goal, but the steps are not at all scripted. I ask what the PCs are going to do, and they tell me what they're doing to start finding out. This quickly lead to an ability check being called for to find out how well that works out -- on a success, it works and the fiction advances a step towards the goal and we reiterate. On a failure, a complication or problem is added and we reiterate. To many failures results in a failed attempt -- but I often use fail forward here so the attempt might yield the goal, but at a steep or unexpected cost. Enough successes leads to the goal with whatever complications have been earned. In this case, the PCs succeeded, but only after a number of failures adding complications. This, organically in play, lead to discovering the aboleth had fled the Outlands, returned to Toril's Underdark (tied into a PC's background), and was going to trade the doodad to some mindflayers for favors (also tied to a PC's background). So, the party now had to organize a trip to the Underdark on Toril, which required calling in a favor to gain a key to access a portal there, and a hike through the Underdark. A few failures on that (I organized that trip as another skill challenge) led to a third party (Drow) being interested in the trade (making the problem four sided) and a few combats but also a few allies were gained. The trade was an epic encounter where the PCs surprised me at multiple turns (I prepped the lair of the Aboleth, and the attending factions, but no outcomes) and recovered the doodad, defeated the Aboleth, made the Mind Flayers aware of them, and extracted a favor of the surviving Drow (who weren't even involved until that session). None of that was prepped, and now the PCs are following up on some of these events, having become convinced the doodad is more than it seems for this much attention (I hadn't planned anything, it was a doodad, but, of course, now it is important as discovered in play), and next sessions are dealing with them following up on that (I don't, at this point, know how).Oh no doubt. You are right. My point was specifically 5e, specifically AP's. And also how it mirrors those that choose to play without AP's.
As far as your 5e game, that's great. You may not know where the PC's choices are taking you except for the next session, but you already have an idea of how they will interact with the next session you created. They may surprise you, and that's always fun as DM. But, if you do already have their motive (which of course has a goal) in mind. So technically even though you haven't written it down, there is an objective.
The short of all of that was that a self-contained side quest (an A with no B, in your parlance) developed into more through play, not prep. There was not B in my head for this A, and I only prepped what came out of the play at the table -- that a skill challenge revealed that the aboleth had fled to it's old lair in the Underdark (I prepped a lair) and that this would require a journey through the Underdark (I prepped some Underdarky challenges to appear on failed ability check during the travel skill challenge). It would be hard to say that this, as in an adventure path, was an A-B-C thing, as no one at the table was at all aware of B or C until it showed up in play.
Now, all that said, do I prep B's and C's sometimes? Absolutely, for 5e. The structure of the game almost requires it. But do I always? Nope, less than half the time, because I've found that I, at most, really only need to prep an A and play will provide any necessary Bs or Cs. I don't count that at all as you present, because these are found in play, not in the GM's notebook.