Signal specifically doesn't want to be decentralized as they don't think it's possible to compete in this market while being decentralized: https://signal.org/blog/the-ecosystem-is-moving/

I don't think Nostr needs to solve the secure chat problem as SimpleX is already doing that. Should just add a filed in profile for SimpleX address as is done with Lightning, and then phase out the current unsafe DM systems. It's a massive waste of developer resources for something that is being done better already + most devs aren't qualified to audit their own creations and security audits are expensive, so if we have lots of Nostr clients implementing a theoretical private and secure DM spec (yet to exist) then we need all of those to get audited, and having lots of implementations also delays progress/adoption of new features, which is especially important when it comes to privacy security, but is not really a problem for a public social media protocol that can easily be implemented by anybody regardless of their skills.