Hi davidpgil!
I'm giving my input here, with the caveat that I'm not a gamer, a developer,
or an artist. Onpon4 is developing a 100% libre game, and will likely be able
to give a more informed opinion.
In regards to making money from your game, significant or not, making the art
(including music) non-libre is not going to be of great use. Apart from
potential ethical issues, there's no way you can stop redistribution without
making the whole game proprietary- in fact, if your game becomes popular
enough, even banning redistribution won't stop it from happening. You can
still ban derivatives or commercial use without stopping the game from being
libre, but neither of these will make anywhere near as much money as
potential career opportunities gained by being able to show a game *you*
made. It is of course your choice as to whether or not you ban these uses,
but there's little to be gained from a financial point of view.
The ways you *can* make money, by contrast, revolve around discarding the
concept of the game as a 'product' and turning it into a 'service'.
Crowdfunding is a great way to do this- set up a campaign with a libre
service (I would recommend CrowdSupply) and offer such things as choice in
how the game is developed, extra material (although this should be libre
too!), or merchandise. By breaking the game down into several
semi-independent parts, such as modular levels, it might very well be
possible to keep the cash rolling in for quite a while. You could also sell
merchandise separately- even if it were something as simple as discs
containing copies of the game.
