There are issues associated with HDMI ARC, but not everyone experiences these issues and it is more likely the implimentation of CEC by the TV manufacturer that gives rise to such issues.
The TV manufacturers don't have anything to do with the authoring or the development of the apps. THis is purely something left up to the third party developers such as Netflix. iPlayer (BBC) and Amazon to deal with. There's nothing to dictate that they have to continue developing their app for the FireTV platform.
Not everyone has an interest in home AV and or cinema, but a considerable number of people do appear to want a big TV without having to also have an AV receiver attached to it. No, the TV manufacturers are not going to alienate their core customers and pander to a niche market by making big screen monitors as opposed to selling what they currently market which caters to both markets. The manufacturers would fail to sell enough of these to substanriate the cost of their manufacture. Believe it or not, you're not the TV manufacturers' target market.
The sale of AV receivers in declining and not on the increase.
I also dispute ypur claim that support for the apps is dropped after 12 months. I've several TVs older than this whose apps are regularly updated. Support with slow the older a product gets and will eventually cease, but this support lasts a lot longer than just 12 months.