I and several fellow audiophiles with good listening skills and systems have spent good money on treating our rooms with all manner of acoustic treatments. In two specific cases folks paid big money for a room acoustics company to come in and professionally treat the rooms. In the end, the rooms sounded better with all the “stuff “ taken out. I experienced the same thing about 13 years ago.
Few absolutes in audio and this includes “treating” a space with all manner of absorption and diffusion. My current room has a few diffusion panels strategically placed and sounds good so I am all for experimenting with treatments. However, I do not believe they are a must for every room and situation. Just not the case based on my experience.
Experimenting and working hard on speaker placement and listening position is something I am willing to say is absolutely needed.