In my mind, TOTL is not “the best” of individuals, but it’s the upper echelon across the “industry.” To me, it’s the combination of soundstage imaging and resolution (not necessarily timbre of tonality). Why these features? Because they are hard to achieve. Otherwise, all the “snake oil” products wouldn’t advertise better resolution and soundstage as selling features.
Why not timbre and tonality? Because what even is the ideal tonality? Etymotic DF, Moondrop VDSF, Harman IE2019? Looking at the successful IEMs, we need to agree that there is not a single “correct” tonality, but a spectrum of acceptable ones. And if tonality is the only thing you are after, just slap a PEQ on it to whatever house curve you want, and you are done.
Edit: why not bass? Again, what is good bass response? Monarch II sub-bass-only? Harman detached bass? Old school big bass shelf all the way to 1kHz? KZ V-shaped bass? And then there is the “bass quality.” It’s just more undefineable mess.
I think “TOTL” or “mid-fi” is kind of hard to define as “what is quality.” Is it an intrinsic characteristic of a product? Is it defined and assigned by the society (community / reviewers / “shillers”)? Maybe both. And at the end of the day, does it matter, practically? Maybe not. Neither price or prestige helped me when I searched for my “end game.” And now I still have no confidence blind buying any of these TOTL.