They want to make it a subscription that starts automatically when you buy the printer. No payment or the linked credit card expires, no more printing. Keep on paying for that subscription each month even if you don't print a single page.
But they're really trying with HP+ printers that come with a 3 month trial of Instant Ink. And it's not like you lose the ability to use 3rd party cartridges, because those HP+ printers already come locked down from factory. Those HP+ printers also have extra REQUIREMENTS: HP Account and internet connection.
It's really hard to break into it. Being accurate enough to print at 300dpi is very difficult, and that's not particularly impressive. If it's color, then the problems are multiplied. You have to precisely align four different print heads (minimum), and the ink needs to be mixed just right for accurate colors.
This is also why you don't see open source 2d printers like you do for 3d printers. On the surface, adding a third dimension seems like it'd make things more complicated, but 3d printers don't need the level of accuracy that 2d printers do.