Well i can agree with this concept but i think it should be a choice that same way you choose if you want to 100% powerup rewards or 50% / 50% rewards for a post. It could be like 50% / 35% / 15% But by choice. Mandating people support a network with out it being their consent is a little concerning and can be seen as a tax. And for a network that prides its self on no transaction fee's this is similar just at a random fixed rate.
But then the developer in me is like omg i could use those funds for some amazing projects. Who decides what the rewards are spent on since this is a tax people will do like they do in real life if nothing useful comes out of it people will complain they are wasting our tax dollars.
The better solution is the 3 way choice split or app owners donating to these causes them selves using money earned from comment_beneficiaries.
If users really wanted to award developers or witnesses they could do it on their own which most don't so the issue is setting up something people don't know about when they first join that they are being taxed on their posts.