How does decentralised finance achieve that? Surely a decentralised market has just as much need for Market Makers, speculation, prop trading and liquidity providers as a centralised one? Those are exactly the jobs banks (and hedge funds etc) currently do right?

I'm not trying to be difficult, I'm honestly asking, has someone changed market structures in a way to eliminate those functions? Things like timed, incremental markets and filling all orders at vwap have been suggested and sometimes tried in the non-crypto space, but I don't think it's ever worked...