I pretty much agree that speakers should be flat but the music should not be flat. The problem is most music is produced badly.
Floyd Toole’s book is documented research on this subject and worth a read (linked below). The short version is people like flat response on and off axis and like some side wall bounce. Now as we are all super into audio we might different just a bit here and there.
When I got into this hobby 20 years ago I read all the tech. I was amazed and drank the cool-aid of brands like B&W. I read the white papers, even bought a pair. Only to have a grating sound and chase electronics. B&W is a good example of making a great speaker sound bad. Now I use speakers with good old paper drivers and big boxes….
My current speakers have 0.5 db adjustments for the treble and mid treble and those 0.5db adjustments make a profound effect on the sound.
I think other things that are hard to measure like dynamics play a huge favor. The measurements will have you thinking a small genelec speaker will sound better than big wilson audios. I can assure you they don’t. The dynamics of those big wilson just sound more real. Something that is missed in measurements. But yes flat measurements are a basic requirement for speakers for me. I will not even audition speakers that are not flat. Then they need to bring the other things.
https://www.amazon.com/Sound-Reproduction-Psychoacoustics-Loudspeakers-Engineering/dp/0240520092