@dwalbert You're definitely not the only one who'd consider doing this. Your question about reversibility is also the whole point! I specifically want the convenience of a notebook/notepad format, but the index card resorting affordance of an index card-based commonplace book or zettelkasten. It's only two thin layers of glue holding them together at the top. In fact, it's the same sort of process and glue that's used on most paper pads that allows you to tear sheets out as you go. As a side benefit with index cards, I'm even less worried about the paper tearing when removing them because the paper is thicker.

Now of course one could also do the reverse process and take a group of cards organized by a particular topic and turn them into their own little mini-book if one wanted. Some writers who use these methods for researching and organizing may want to do something like this when their book project(s) are over for archival purposes, though simply keeping cards in their boxes is probably just as convenient, though time and entropy may manage to re-organize those cards depending on one's wishes.

I had started out looking for companies that made something like this, but realized there aren't any and that I could just as easily make my own with the exact cards I prefer for a fraction of what I would have been charged for the privilege.