Actually, it doesn't take much away from steem. The steem could still be farmed by witnesses and we could make it so node operators also can mine steem and sbd.
Just kill the inflation to below 5% and golden. This doesn't make the token any harder to implement as a currency in sites. It makes it more likely to create buy pressure. As it has on eos and eth.
Especially if people need to buy steem to get smt's and power up for smart contracts.
Proof of brain is much better than mining to distribute a currency to everyone. Not everyone can or wants to mine.
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It would be cheap to mine. A node with mira is like 8-40 dollars to run.
Also proof of brain is easier to abuse. The smt's would still be pob. Also as you said no one want's to mine. Pob is mining, and they could still mine by earning SMT's and trading them for steem.
PoB is a form of human computation where the brain is the mining device.