Decentralised exchanges already work, but they cannot play in the same league for trading speed.

You cannot decentralise bank transfers without friction. There is P2P trust and other considerations.

Slower trading as standard would probably benefit BTC at this point, but as long as there's faster and more efficient trading, it will cannibalise the market.

My prediction:
MtGox will improve their performance and for the time being things will continue "as usual". I don't think this is going to change any time soon.