traders won't put up with it. they want a centralised ledger for the speed and the walls.
and no one's come up with a satisfactory answer for fiat yet in the context of a trading exchange as opposed to a straight fiat to btc exchange.
Correct. People keep focusing on decentralized exchanges, but they don't realize that there is no way they will be trading in the same way, as centralized exchanges allow them to trade. In case of Bitcoin, it's even worse due to its confirmations, that even transactions with a well sized fee might take 30-40 minuts to confirm. It gets the job done, but don't tell me how.
Bitsquare for example offers a wide variety of fiat related payment options, but this puts its decentralized aspect to waste. Fiat is just impossible to incorporate into their platforms in a totally decentralized way. Tether is also no option since this token has [1] lost its purpose of maintaining a 1:1 value with the USD, and [2] it's insanely centralized.