From your original post, I think you pretty much understand the trade offs:
-Steam Deck is cheaper but with limited flexibility and future-proofing.
-Laptops are more flexible, capable, and future-proof, but miniaturization can have implications for cost, thermals and noise.
-Desktops are the ultimate in flexibility, capability, future proofing and cost effectiveness, so long as you have the space needed and patience to research and design your ideal platform.

Personally, I prefer desktops, but then I don't use a TV/Couch/Controller. My current PC is 10 years old, with one mid-life GPU upgrade and a switch from HDD to SSD. Total cost over those 10 years would be sub-2000$, and it still plays most games well. This year I intend to move the SSDs and GPU over to a new motherboard/CPU/memory, once I have decided what best fits my needs. Unfortunately, I don't expect this to be as cheap as 2012, but we'll see how the current market slowdown affects pricing.