Not everybody want to use their phone, core2slow. I don't. And I know for a fact my parents don't want to either. Most people like the simplicity of picking up a remote and pushing a button, instead of having to do some arcane magic ritual to get their phone to hopefully connect to this "caster" thingy and hope it works.
Not to mention, if the phone isn't connected to Wifi it's going to use their data to steam Netflix and if they don't have good service at home, it's not going to be HD.
You really have to stop talking about Chromecasts. You don't know how they work, at all.
Any device is able to connect to a Chromecast
if the guest access is turned on and the person is able to see the TV screen. It's not an option turned on by default.
A Chromecast will NOT eat your mobile data, EVER. For a Chromecast to work, it has to be connected to a WiFi network. The only way your mobile data gets used is if you are hotspotting with one phone (or a dedicated hotspot) and using another to actually cast.
As for your first paragraph, no disagreements from me. Different strokes. People who don't get that need to make an effort to understand that they are not the universe. There are very few cases where that mindset plays out well enough, but generally it just boils down to preference.
I wouldn't mind upgrading my Chromecast to an Android TV device down the line, specifically a Shield because of the PC game streaming.