The iOS app definitely needs chrome cast support.
i think for now every of the big players will create their own apps/boxes - i imagine "prime TV"
at some point the fragmentation will annoy customers so much that they will find ways to combine - but that's in 5-10 years
Chromecast is so cheap that its probably putting downwards price pressure on the devices themselves. And I don't personally have any issue flicking between all of my various "set top" boxes: chromecast, WD Live, Apple TV, old school dvd player, Tivo. After all, what issues does this fragmentation cause, apart from a bit of a mess of HDMI cables, and a shortage of HDMI ports on the TV?