You could have it both ways, but there are quite a lot of kanji that are reused as radicals later.
It’s easier to say “you know this already” instead of trying to “see the fault” in 失. There is some background on the “radical method” that the radicals should be concrete objects that can act instead of abstract concepts. You should “physically see” what the radical represents. So a husband and slide is better at first, and when you know a kanji well enough you can switch to the more abstract meaning.
WK is not really emphasising this, but I think they try to use this. 夫 is actually a pictograph of a man 人 with some knot hairstyle that only married men were using in ancient China, when you know that it’s easier to actually “see” in 扶 a husband helping with his hand.
They should add the kanji that inspired a radical to the lists of “kanji that use this radical”, though.