One thing is that the desktop PC itself has become a niche product, mainly used by gamers and content creators. I would expect the prices of hardware to go up if the market for desktops (and especially custom builds) is shrinking.
I have had a 55" TV on a larger desk at about 4' away, and find it comfortable for both games and productivity. I'm in the process of moving to a 77" model on a mobile stand and use it from 7-8' away. I find a couch or recliner with a small table for the mouse/KB (which has casters and can be moved out of the way as needed) more comfortable than a traditional desk, especially after sitting at a desk at work all day.I too, have a PC in a HTPC type arrangement with a 4K TV but am not as productive on it. The question of using a 55" (really any larger than roughly 42", IMO) TV is more about other ergonomics. It's too big to sit directly behind at a desk, and productivity (at least for me) tends to tank if I'm sitting on a couch with a keyboard, at a normal distance at which you'd use a 55" TV, which is a bit too far away to see minute details that I often work with.
I suppose you could build a custom cockpit sort of arrangement that makes everything be where it should for productivity but then it is not as good a setup for normal HTPC use.