"Mp3 players" will survive, not because they are cheap, but because audiophiles need them. If you have an expansive HiFi headset, you just can't use it with a phone. You'll want a player with a good dac, and a wired connection to the headset to listen to your HiRes music.
And "Mp3 player", which are actually called music player because they are now used more for HiRes music and not mp3, are not cheap. Products from Sony, Cowon, Fiio, Astell & Kern can cost more than a thousand euros.
It is a niche market, but a niche market that will stay for long.
It's like digital cameras. Phones can now take very nice pictures, but digital cameras have not disappeared, and people buy them not because they are cheap