That is a profoundly good idea. Let the TV manufacturer bundle all their "smart apps" stuff on USB-powered HDMI stick like a chromecast. Then five years down the line when the manufacturer has completely forgotten that the model even exists you can just spend $100 and get a modern one from some third party.
With HDMI CEC (Consumer Electronics Control) they can even integrate controlling the HDMI stick through the TV's remote for a completely seamless experience.
It's probably one of those ideas that is so good no manufacturer will ever do it though and we'll just be stuck with obsolete and forgotten software.
I still don't understand why "AppleTV" isn't just a dock for your phone with an HDMI tail and an IR receiver on the front... Maybe it's old fashioned of me to not use my phone while watching TV.
Chromecast has shown how very little you actually need connected to the TV in order to provide this kind of functionality though.
Personally I'll stick with an external device, as it means there's an endless line of companies waiting to pick up the baton, and if I don't find an off the shelf device, I can set up my own.
There's just no compelling reason to lock myself into whatever the TV manufacturer decides to provide.