ph0rk
Friendship is Magic, and Magic is Heresy.
I'd argue the opposite - I don't think spells are required to do things, but rather that because spells exist to do things players (and DMs) assume that is the only way to do them.I don't see that it needs to be tightly quarantined so as to make spells essential to achieve goals like travelling to other parts of the multiverse.
Spells let X just happen.
Yep, and in a world where a spell can get you to plane shift, it is completely natural for players (and their characters) to not even consider finding a portal the "hard way". This is a lot like the situation described in the Asimov story The Feeling of Power where even scientists had lost the skill of basic arithmetic, because computers did it for them. Magic is like technology in d&d, and plane shift is Uber and GPS all in one. Could the players get there without it? Sure. But it seems easier for them to find someone with plane shift than to figure it out without it entirely.
I agree that the problem as described is, ultimately, plane shift.