"Flat" interfaces aren't bad because they lack an ineffable whimsy of embodied human experience, they're bad because they threw out the baby the bathwater, tossing decades of conventions and hard-learned accessibility lessons in the name of supporting a touchscreen.
Compared to 20 years ago, everyone is shipping half-website-half-desktop abominations (e.g. with Electron[0]) and reinventing UX wheels. Too many apps/sites impose "their own look" instead of following what the user has already learned. [1] Often users must guess whether certain things are even clickable, how a certain toggle looks when enabled, whether certain settings are a single-select option or a multi-select tickbox... And memorize those rules with per-app or per-website granularity.
> You can talk while clicking, listen while reading, look at an image while spinning a knob, gesture while talking.
Those are all things people do after "make computer do what I want" has become automatic.
Now when--for example--trying to find the 21st item they just added inside a list that is vertically limited to 20 and the custom grey-on-grey scrollbar is always hidden unless you've currently hovering a mouse exactly in the right 5-pixel-wide strip between two columns of the interface.
[0] A sample listing of software readers may be familiar with: https://www.electronjs.org/apps
[1] That may be due to deliberate "remember us" branding, whatever was fastest-to-ship, because things to look new to get somebody a promotion, because they want to create a switching-cost so current users feel bad trying to use a competitor's product... Or because someone like the blog-poster has misguidedly tried to make a "richer experience."