If the system were to rely on donations exclusively, the rate at which proposals could be funded would be complete unknown

The rate will still be unknown given the fluctuating (sometimes extremely so) Steem price. Maybe somewhat less unknown than with donations, but in some cases it could even be more so (donators might contribute more, in STEEM units, to pay for important work if needed to make up for a falling Steem price)

I still don't think donations alone are a good approach.