But if few people know that 99.9999 == 4m19s, then how is anyone losing out on changes? If you hadn't told me and I didn't have a need to calculate it with certainty, I'd have believed you if you'd said it was an hour a month.

I think the point is that the metric being meaningless right now to most people makes it easier to shift the reliability-Overton-window. 99.9999 vs 98.0 seems not to bad to a layman (I.e. executives), but 4m19s to 12 hours seems pretty intuitively bad. Sure, we may lose some shift in the immediate future, but it's easier for that to continue happening with just percentages, is the point.

It's not like we have to stop showing percentages as well, but time is a much more accessible expression. Right now the lay-ness of company leadership has already allowed the shift to happen pretty markedly.