That's where you want to consider second order effects. The type of person who's buying an Apple Wifi router is probably spending a lot of money on Apple products, it's part of the "long tail". They might be the people most likely to evangelize their products. It keeps expertise about wifi inside of Apple. Getting rid of it seems like a move made by an executive who had to "do something" to look impressive. Apple isn't a cash-poor company that needs to ditch every product that's not part of their core offering. And the Airport wasn't a failed product line, it was a beloved one.

I have a feeling it's more to do with not wanting to help technologically illiterate individuals set up their own wifi, who blame "Apple" when their wifi doesn't work. But of course that's just speculation. The Airport lineup was bar-none the best easy-setup home router on the market when it was pulled.