Those stores are a mess inside and I can only imagine make a profit because they sell everything. In central Tokyo, most of them are just tourist traps at this point.
Many years ago I lived in Shibuya and Donki was the only place I could get a lot of basic supplies, including even groceries, at a reasonable price. Now I live somewhere else in central Tokyo and don’t go nearly as often thanks to Amazon, but it’s still the only place to buy a lot of items.
Also my comment doesn't preclude Donki having some daily necessities. As for calling it a "staple" for central Tokyo, that's a bit of a stretch. If you said outside of the 23 wards, then I would agree.
Go see a Donki in a residential neighborhood and it'll be full of locals. I live in Shirokane, an upscale highly residential neighborhood off of Meguro station, and even if it's reputed as one of the richest place in Tokyo there is a Donki and it's packed with local families shopping.
I admit I am biased in that my nearest Donki is the Roppongi location.
You’re overgeneralizing from limited experience. Not everything in Japan is like Shibuya or Roppongi.
I'm well aware of the diversity (or sometimes lackthereof) of Japanese cities/neighborhoods. I've been living here for 10 years.