Let's start with an observation: these crusts are generally as hard as wood!
You've done all you can, but it's impossible to grate more, especially by hand - too hard, and you risk injuring your fingertips with the grater if you press too hard.

Should the crusts be thrown away? No, that would be a real shame, but how do you use them? Here are a few tips.
In a hot, liquid preparation
It's very simple: as soon as you prepare something hot and liquid - a soup, gratin or sauce, for example - add your croutons as soon as you start cooking, and let them flavour your preparation deliciously throughout. Remove them before blending (soups) or using, noting that they have become soft and can be eaten.

Here are a few examples of recipes where you can use these croutons:
Potato gratin (in the milk with the potatoes),
béchamel sauce (also in the milk) or
leek-potato soup (when you add the water) or
parmesan-flavored gratin.
You can also prepare a
parmesan milk that you can use later in a recipe.
In the oven
You can place your croutes in the oven for a few minutes at 180°C (360°F), where they will puff up and become crispy, ideal for serving as an aperitif, for example.

Parmesan olive oil
Cut your croutes into small pieces, place in a jar with a lid, cover with olive oil and leave to macerate for at least 15 days.
The resulting Parmesan oil will add a pleasant aroma to your sauces or pasta dishes.
To sum up: Parmesan crumbles should not be thrown away - it would be a pity. It's better to use them in cooking, either hot (soups, sauces, in the oven, etc.), or cold (macerated in olive oil).