I agree and disagree.
The DM's job includes the creation of NPCs, and those NPCs can and should attempt to manipulate plot points on their own. The robber barons step up their attacks; the king sends guardsmen along the road with the tax money. A notice goes up with a reward for the capture or death of the robber barons. If the PCs take the challenge up, the king expects the tax money to be returned -- failure to do so means the king's next notice is for their capture or death.
Meanwhile, the orcs of the Grumbling Wood have been opening a portal to some fell realm, urged on by human and half-orc cultists. This was an alternate plot thread that the PCs could have followed up on if they hadn't chosen to bring in the robber barons instead. So, in their absence, the Band of the Crimson Swan tries to stop the orcs and fails. The lone escapee returns with whispered tales of eldritch horrors and the sure knowledge that her captured companions will be sacrificed at the full moon -- unleashing a terrible evil into the material plane -- unless someone is willing to return to the Grumbling Wood with her to save them and stop the orcs.
OTOH, there is a caravan to Ortuga that is hiring guards, and the PCs' success with the robber barons has made the caravan master seek them out to either try to hire them, or to consult them on who to hire.
Also, there are rumors of an ancient dwarf hold, long abandoned, having been discovered in the nearby mountains, and the PCs know that other groups will be seeking the place to try their luck.
Everything the PCs choose has consequences....some good, some ill, some both. Plot, IMHO, is the interweave between action and reaction, choice and consequence. Things happen without the PCs. They get better or worse because of what NPCs do or do not do while the PCs are away doing other things. The caravan master they helped outfit with guards sings their praises in far Ortuga. The orcs are thwarted because the PCs see this as a more immediate threat, and the Band of the Crimson Swan tries to recruit them or offer them a landhold. Meanwhile, rumors begin to warn the PCs about what they might find should they go to the dwarfhold, and the gear of the fallen add to the treasures that might be found there.