Segregated witness is a complete change of the security model of bitcoin. Why would anyone want to do that? The previous security model has worked perfectly to secure payments for users and merchants so a reasonable explanation is that it is not in the interest of users and merchants to completely change the security model. Actually, it only seems to make sense for people who are deliberately trying to undermine the decentralized element of bitcoin, ie the powers that be represented by Blockstream. Blockstream has managed to literally block the stream of bitcoin payments by insisting on keeping the 1 MB blocksize limit. Now segwit might for example succeed in being the first step in creating one or more centralized solutions on top of bitcoin but who would want to use that product? Not the traditional users of bitcoin. With centralized solutions on top of bitcoin we can expect that those centralised solution might look attractive in the beginning but eventually centralised solutions will be abused by the parties controlling it. Centralisation always ends that way.