A mandatory fee is the only one that guarantees funding. The foundation should at least consider a low mandatory fee (at least 5%) with an optional opt-in for additional contributions. If not they should consider alternative sources of funding.
Miners will only contribute if it is directly in their self-interest. There is no guarantee that the value will go up over time. Most miners sell very soon after mining. If the value goes up miners will be up against even more competition as more miners will enter the space. I don’t see any real incentive for them to contribute significant amounts of their income.