@Kumasasa
I'm sorry that the bug was a duplicate, I searched everywhere, and the bug you linked didn't contained any precise keyword that would have helped me, and I still think my report is better worded and documented.
However, this clearly doesn't work as intended, as having the possibility of such a ridiculous memory leak cannot be intended.
@OscarParker Simply because a hopper is unable to do the same thing on a normal chest.
@Grum
This shouldn't be tagged as "Work as Intended", simply because of the danger of clogging the server.
Invulnerable entities too will never stop falling, causing major lag in the case of, for example, an invulnerable entity spawner. It is virtually impossible to get rid of an invulnerable entity in Vanilla Minecraft. Some entities will despawn naturally, however many don't even have an option to decide if they can or not, like minecarts.
I tried to have moving invulnerable minecarts in a mine, but I'm forced to make them return to the first point, and to use MCedit to delete them. I often have to export every chunks to delete entities below y=0, because MCedit doesn't allow that.