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.
RE: Part 3 of Our Plan to Onboard the Masses: Smart Media Tokens