Roots dying from the plantbase down?

[D_novice]

I've seen this with phals and cymbidiums. And, to be sure, I don't even know if they're dying. But it would make more sense to me if roots died from the tip, back towards the plant. How can a dead base of a root continue to send nutrients out to the tip? It makes no sense to me.

The latest victim is a Phal gigantea that it would be a crying shame to lose. Even though it's not a large plant, it wasn't cheap (by my standards).

I don't know that there would be any fix for this, probably if this plant is sick it's a goner. But it's leaves look mostly healthy.