No idea about what the future will bring, but I agree that it doesn't look too promising, given that, for instance, the Z was out in the US for less than a year (even if the F seems to be a shrunk Z). I wouldn't make the jump to a phone, even if Sony merged the Walkman and Experia lines. I don't want to be stuck with bill for features I don't need and much prefer to have a dedicated DAP. If they stop selling Walkman, I'll look out for other, less mainstream, stuff (perhaps Cowon).
Still, the F and E series are being sold in the Sony Store (US). They haven't been taken down (e.g., http://store.sony.com/webapp/wcs/stores/servlet/CategoryDisplay?catalogId=10551&storeId=10151&langId=-1&categoryId=8198552921644871536&N=4294940721).
Let's keep out fingers crossed that Sony doesn't pull the plug on the DAP Walkman. Apple sells millions more, but they seem to keep investing on new stuff (iPod Touch 5, etc.).