I mean it's just the literal mainline. It's like saying you're giving up the dragon because of that "one line" where white plays 13. h4.
13.a4 is the "Main Line". Not 13.Rc1.
Historically, yes, but my impression is that 13 Rc1 has largely supplanted it — for pretty much the exact reasons you gave.
As Black in the Najdorf I typically leave my King in the center for a long time, or even forego castling entirely.
That's true in certain variations of any opening, not just the Sicilian.
Same can be said for the Advance French with 6...Nh6. Black delays or foregoes castling outright.