Element/Matrix/Gitter is a great IRC-replacement and public chat, Mastodon is great as a decentralized Twitter alternative, IPFS is great for decentralized public file sharing. Default-public services like this probably have an easier time being decentralized. It'd actually be nice to see thoughtful integrations between these three IMO.
The greatest advantage Signal has is probably doing same-day updates for all clients while being dead simple to use well. A decentralized protocol could achieve these things, but none have yet. Still I hope someday soon we can have everything be decentralized, but like the blog post says, we can't let the perfect be the enemy of the good.