Dev Fund Proposal: opt-out mechanism for sharing block rewards by miners for dev fund

Advocate: dontbeevil

(Obvious) Problem: Block reward distribution to ECC, Zcash Foundation and others ends sometime in 2020.

Solution: Instead of simply extending the current mechanism of block reward distribution (only 80% going to miners)- this time enabling miners to (partial or full) opt-out of sharing their block rewards for dev fund. So, protocol can be changed to allow miners to signal sharing of their block rewards from MIN to MAX percentage.

MIN could be as low as 0%
MAX could be as high as 20%

Edit #1: Why introduce MIN and MAX: As price of zcash fluctuates, it helps miners adjust amount of block rewards towards dev fund for continued development and improvement of zcash protocol.
If zcash price goes down 2x (during bear cycle), then miners have the power to increase block reward share to MAX, for continued development.
Similarly, if price of zcash goes up 2x (bull market), they can reduce the reward percentage without affecting USD value.

Both MAX and MIN can be configured to decrease every 4 years at block reward halving event.
Reasoning: In the long run, Zcash protocol development becomes more decentralized and will not just rely on contributions from Zcash Foundation and ECC.
Assumption: Avg. value of Zcash protocol increases with continued development, in the long run.