While I don't wholeheartedly disagree, I think this sort of appeals to the authority of the books and/or rules in a way that is unsatisfactory.
I don't see how or why you would claim that.
It suggests that the players should always have perfect information and always have the right tool. That just isn't true.
And I understand where you got that idea even less.
Now in this case I started with a premise -- the PCs can't simply escape out of the Faewild with Plane Shift --
Right. That's what makes it heavy handed railroading. The premise I would start with is probably more like, "Given this villain, what stat block would they have? What minions? What resources? Now how would they allocate those resources to some task?"
That they could perfectly seal the border would never occur to me. I probably wouldn't be allowing that for something less than something with Divine Rank, and unless the PCs were very nearly divine rank themselves I'd never start with the premise, "These are villains the PCs should be doing anything about." I wouldn't put the PCs into that contest because I wouldn't be interested in it. Sure, in theory the PC's could have come directly into contact with Keeropus the Archmage at 1st level, but that wouldn't have been particularly interesting. Instead I'd try to come up with what they could do that would be fair or reasonable. Generally this would be determined by, "Would I ever consider granting the PCs the power to do this, should they obtain similar level?" If I wouldn't let the PCs do it, the NPCs don't get to do it either.
Which is why I'd never take any object from the PCs without a saving throw. And saving throws can always be passed on a natural 20 at the least, so if I'm giving it a saving throw then I'm suggesting an out - a way my plans could go wrong. And note that in my game, hard saves are rare. It's not like 3.5 where saving throw DCs routinely just scale up into the 30s or 40s, and for the most part I couldn't justify a save that high within my rules (maybe a Greater Deity working within their portfolio).
But, as you say, all this is in service to the scenario and not a "natural" outgrowth of the rules or lore.
Right. Which is why it is to me very heavy-handed railroading. You have a particular scenario you want to force and you are figuring out how to do that.
Now of course, GMs have unlimited fiat and resources. You can always fill in the sort of background that would let the thing be reasonable in the setting. Artifacts created by deities or near deities would be the sort of thing that would justify NPC of level X having access to powers of level X+Y. But then, artifacts immediately become weaknesses, things that can be stolen or which have drawbacks or limits. The process I'm talking about of fiction first to me produces different outcomes than meta first. And it tends to leave lots of opportunities for your plans to get wrecked.
And if your plans don't get wrecked, you are railroading. In fact, as a player that's one of the ways I detect whether a GM is fudging or otherwise using illusionism. Honest games always go off the rails at points, and are less fun than imagined or are less well paced than hoped or more difficult or less difficult than envisioned without balancing adjustments suddenly appearing. Whether you consider this a good idea or not depends on your theory as a GM. I know a lot of GMs online and in the community that are like, "You want to run the most fun games, just throw illusionism at everything." And that works, just as long as the veil doesn't get torn.