If the government actually cared about the environment, forcing the creation of unnecessary waste like this would be illegal.
It's unlikely HP would bother with the envelopes if it wasn't cheaper for them then building new cartridges.
If they were just going to throw them away, they'd do something like team up with Best Buy to have an cartridge recycling bin available. Most of the PR value at a fraction of the cost.
Actually sending prepaid envelopes means they're probably recycling them.
If:
A) You have subscribed to HP Instant Ink and B) You stop paying for your subscription
The ink you have in your home as part of that subscription will no longer work in your printer.
You can use other ink in the printer. The printer is not bricked. It simply disables the revocable license you were paying for to use the ink in the cartridges they shipped you. They sell cartridges that don't have this model.
Whether or not the model is dumb or the idea that the ink "stops working" if you stop paying makes one chafe, the printer itself does not magically turn into a paperweight. (And the ink is supposed to be returned to HP.)
With so many issues affecting humanity, it's impossible to address them all and immediately unless you live in an authoritarian yet super-efficient regime. If you like that, look into Singapore.