Side chains are centralized I.O.U.s that still need bitcoin transactions in order to be fulfilled.  They remove the benefits of using the decentralized bitcoin network while still using the network.<p>If 10 m people used side chains (just for today), it would take some of their transactions up to 40 days to fulfill.  If 10 m used it again (just tomorrow), it would take almost 3 months for some transactions to fulfill.<p>Most merchants will not wait that long for an I.O.U., especially with fluctuating BTC exchange rates, making side chains much worse and less competitive than ACH.