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Just to give you an example of why BIP 100 is so flawed, imagine for a minute that we aren't talking about block size and imagine that instead we're talking about the block reward schedule. Say that BIP 100 would give miners the ability to vote to accelerate or decelerate the halving process. If miners voted, they could keep the 25 BTC rewards going for 8 years or 12 or more. They could do this without merchants or users having any say, besides introducing a new hard fork taking the vote away from miners. Does that sound like a good idea?
Or to give another example, lets say a BIP666 comes along to set the miner reward per block to 1,000,000 BTC
Of course all miners would vote for it since they are all stupid morons who only want 1,000,000 BTC per block.
Yes my example is as silly as yours.
You need to notice the blatantly obvious ...
Miner rewards depends on the health of bitcoin.
Most of them aren't stupid enough to come up with, or vote for, either your example or mine.
... and ... miners confirm blocks and transactions, no one else does. That's how bitcoin works. If you don't like that, then you are in the wrong place.