That's not necessarily toner left in the cartridge. It could be developer. Laser printers have 2 separate forms of powder inside of them. One being toner, and the other being developer.
The developer has fine metal particulates inside that "charge" the toner, enabling the toner to be pulled off of the drum, and onto the page.
Larger copiers have the Developer, and toner separate. However, the cartridge pictured is the full process unit.
Developer is, in all modern machines, a roller. It lasts almost forever - far longer than any home user will use their machine at least.
When it does run out, I don't think it's due to wear or the metal particles being 'used up' in any way, but instead because a layer of dirt has stuck to the surface of the roller, so it is no longer any good at transferring toner.
Developer loses its charge over time. You have to replace it.
Every laser copier/printer still uses developer. If you can show me a model of laser printer that does not, I'm all ears.
There is also a transfer roller in each copier, maybe that's what you're talking about?
I could be mistaken, but as recently as 2020, every commercial copier I worked on, still uses developer. I have never worked on, or heard of a laser copier without developer.
"Mono-component" toner is toner that is magnetic in its own rite, so it doesn't need separate developer (iron powder) to make it magnetic. I believe the original mono-component toners had magnetite in them, but I believe now there's non-magnetic mono-component toner too.
Magnetic printers, with magnetic toner, have been built [1] but never really caught on.
It was one of those dead ends from the early years of trying to build a faster printer.
Somehow I managed to encounter, early in my career, the first CRT phototypesetter prototype (Harris-Intertype), the first electrostatic liquid printer prototype (Clevite-Brush), the Data Interface Magnetic Printer, the Teletype Inktronic (swept a beam of ink dots with deflection plates, like a CRT), and a Corning Glass display that used photochromic glass (write with UV, view in green, erase with IR).
None were successful products, although for a decade or so, electrostatic liquid printers from Versatek [2] were a thing. They used "dark juice", toner particles in a liquid suspension. A row of electrodes charged the paper, which passed over a liquid surface where the paper picked up toner particles. Then a heating station fused the toner and evaporated the liquid. Required special paper. Print quality was mediocre, but the devices were quiet and fast.
In classic xerography, yes, the toner is moved around electrostatically. But it also needs to have magnetic properties (either intrinsically or by means of developer) to form a magnetic “brush” which is presented to the drum. Think of iron filings mixed with toner on a piece of paper and a magnet on the other side. The right static charge will be able to pull the toner out while leaving the iron filings behind.
Also it's not flat across the top so it's probably more like 10-12% than 15%. They measured the highest edge when there's a pretty significant slope compared to the full toner cartridge.
The developer has fine metal particulates inside that "charge" the toner, enabling the toner to be pulled off of the drum, and onto the page.
Larger copiers have the Developer, and toner separate. However, the cartridge pictured is the full process unit.