The problem is that people prefer convenience over safety and security and sadly, the decentralized exchanges are not as newbie-friendly and easy to use as the centralized ones and that's exactly what pushes people away.
it's not just about security/third party custody. it's about inability to enforce KYC. the USA is even going after bitmex now (even though USA traders have been banned there for years), apparently for allowing americans to access the site via VPN. if the american government has their way, all the biggest margin and derivatives exchanges will be forced into mandatory KYC. that could make decentralized margin trading all the more attractive.
there's definitely a big niche for this but i'm skeptical blocknet is anywhere close to production ready code. it could take several years to make this happen, even with a clunky, buggy interface.