And if I read rightly the NPC villains are only reactive, not proactive, once play begins?
NPC villains are frequently proactive, especially at the start of the campaign, when the PCs haven't done anything for them to react against. Many a campaign has been kicked off by the Big Bad capturing a royal heir, or stealing the magic crystal. That the forces of good will fail to hold off the forces of evil, unless the PCs intervene, is part of the premise which makes the campaign worth running. Given the multitude of possible worlds that we
could play in, the world where the PCs
might make a difference is much more interesting than one where good or evil will prevail
regardless of their choices.
But that's all part of the premise. It happens entirely
outside of the game. Choosing which
premise to start from is like choosing which movie to watch or novel to read. We can only judge the integrity of a story by how well it
follows from the premise.
And, just one storyline or plot arc - kind of like an adventure path?
It's also (ideally) played with dice, much like an adventure path. The numbering of plans that are simultaneously in motion has no significance upon whether those unfold organically or through contrivance. You can railroad three plot arcs as easily as you can railroad one, as long as the DM insists on biasing their outcomes.
And even in your ideal game noted above there's still specific events and NPCs and stuff the party has to (or is certainly expected to) deal with, the only apparent caveat being that they are put in place before play begins rather than after. But the characters still have to go to places a then b then c and do things p then q then r to prevent events y and z from occurring...which on the face of it sounds more railroady than baiting hooks at the right moment.
In most such games, there's
probably going to be a most-obvious path that will get the PCs from point A to point Z, but they don't
need to follow that path if they have any better ideas. You don't
need to go take out the Dragon in the Dragon's Den before you confront the Big Bad. You don't even necessarily need to
deal with the Dragon, or befriend it, or acknowledge its existence in any way.
The players have the freedom to do whatever makes sense to their characters. The only limit is the actual capability of their characters to affect change within the world. The DM is operating under the same constraints - their NPCs can do whatever makes sense to them, limited only by their individual agencies (and motives) within the world.
Another thing to keep in mind is that not all railroading is done by the DM. Sometimes the players railroad themselves in that on finishing one adventure they have already decided what the next one will be whether it's what the DM had in mind or not.
All railroading is done by the DM, because the DM is the
only one with the agency to declare what is true within the world. The player
can't decide that there is a nearby pitcher in which to catch blood, or that the one artifact they happened to find was also coincidentally the one that was being sought by an archaeological dig several days away, unless you're playing in one of those weird hippie games where that's a thing (usually moderated by some sort of metagame resource, like plot points).
In D&D, nothing is true unless the DM decides that it is true. To do so, the DM can either make a judgment call based on known and unknown factors and possibly roll some dice to account for their own uncertainty, or they can just pick an outcome based on what they
want to happen. In the context of this thread, as posited in the opening post, the latter method is railroading. It doesn't matter
why the DM chooses that outcome; if they're doing it for any reason
other than as a judgment call on what
should naturally happen based on existing factors, then it's railroading.
If the players decide to follow the plot in a certain direction that you did not expect, you have three basic options as the DM: 1) You can railroad
against them, forcing the plot back toward your original path; 2) You can railroad
with them, forcing the plot to develop in such a way that their new path will be interesting; or 3) You can make a judgment call about how that path
would look, based on existing factors, and let it play out on its own merits.
Most people in these forums are likely to say that (1) is bad, because it takes away player agency. Some people might say that (2) is good, because (1) is bad and (3) might very well be boring. All three choices have their own merits, but the fact remains that (1) and (2) are
both cases of the DM railroading the players in the direction they want; the only difference is
what the DM wants in each case.