HP Instant Ink is so widely misunderstood that HP should really just cancel the entire program because it's been an absolute PR disaster ever since its inception.
With Instant Ink, you are not subscribing to ink cartridges. You are subscribing to a monthly quota of printed pages. The ink is merely the vehicle to deliver the printed pages. You never own the ink cartridges. At best, you're renting them. When you decide you don't want to pay the rent anymore, you don't get to keep using them.
And it can. You just need to buy a regular cartridge now.
You purchased the printer. You licensed the ink cartridge, via pretty clear terms.