GPL software is a box that must be kept open, so that everybody would be able to take from it.

If you pick the box and build an altered version of it, you must keep it open, you are legally prohibited from attaching a lid to it.

There's nothing about any expectations, let alone obligations, to put anything back into the original box. Usually it's not very easy (you must follow strict standards) or even impossible (see e.g. SQLite).