That's right: even if the dipole field diminishes, the secular field is left -- there's never a time when the field's zero. True, the non-dipole field is highly changeable, and would offer a patchy coverage, but it would still help us to ward off solar storms.  And there's no evidence at all, looking at the past record of species extinctions, to statistically match up a field flip with the destruction of life.  [Of course, destroying our fragile human civilisation is another thing entirely!]

And it's also true that we misinterpret the meaning of 'sudden': to a geologist, 10,000 years is almost instantaneous!