@davidrevoy >they can have a viral effect
The freedom doesn't spread like a virus - someone has to take the explicit choice to take part of the spider plant and have the freedom grow.

>potentially requiring derivative artworks to be open-sourced.
The AGPLv3 is a free software license - it does not say "open" in it, or require such a thing.

For derivative works, only people who receive a modified derivative work or has their computation done by such modified version needs to be provided a copy - you can choose to not provide a copy and not do some users computation and if so, you don't need to give that user anything.

I don't see how it would be a bad thing if artworks were released under a free license, but under section 7 you can add an exception the the AGPLv3-or-later header that ensures such doesn't result;
"As a special exception, if you create a artwork which uses these brushes, and/or embed these brushes or unaltered portions of these brushes into the artwork, these brushes do not by themself cause the resulting artwork to be covered by the relevant GNU Affero General Public License version. This exception does not however invalidate any other reasons why the artwork might be covered by the relevant GNU Affero General Public License version. If you modify these brushes, you may extend this exception to your version of the brushes, but you are not obligated to do so. If you do not wish to do so, delete this exception statement from your version."

>but might not be ideal for creative assets.
Please do not devalue your artworks by referring to them to something as interchangeable as "assets".

The AGPLv3 is suitable for anything that is copyrightable with a source form.