I installed signal and it worked. I told a friend to install signal and it worked. I told my mom to install signal and it worked. The interface was basically the same. Any friend who installed it appeared the same way they would appear in WhatsApp. I didn't have to teach any of these people anything to get them to use it. I didn't have to talk them into making an account to use it. That is what I mean by drop in.
It's not a drop in for the behavior (talk to other people who use Facebook owned services in a way where Facebook can read all of your conversations), it's a drop in for interface (communicate with others who use the app in the same way you communicated with others using WhatsApp).
I just spent time looking at matrix.
google "matrix"
oh right, name space collision with popular 90s movie
google "matrix app"
oh, this is some library or something, not an app
searching the page for client. "Maybe under matrix live?"
see clients button in sub menu
see 10 plus options I don't recognize the name of and immediately lose interest
search "matrix" on app store
see apps with 2 stars or less than 10 reviews, nothing official.
Matrix is what you get from the people who say "isn't Dropbox just rsync?" "isn't a chat client just a GUI for a protocol?"
100 bespoke solutions to the same problem (10 different desktop clients) is an engineering nightmare and it robs a service of "economies of scale" enabled improvements.
I don't want to read a wall of text to understand something and neither does my mom. We want to search a keyword (or "best chat app"), download an app, and use it for its purpose. That might not be optimal, but you won't see wide spread adoption without it. If you tell me "matrix is best for chat" and I can't search for matrix on google, one click download a client, and be chatting with another person who did the same with minimal setup, it's not just a non starter as far as getting widespread adoption, but it's very far from "drop in." I don't personally have a single friend who has asked me to use matrix/a matrix client, or told me how awesome matrix is. Matrix supporters should ask themselves why the main place you ever see Matrix mentioned on hacker news is in posts about Signal.
Protocols don't win customers, user interfaces do. The average end user wants to download an app and use it without having to understand what "federation" or what the security implications of something is or who owns what data. They want their most knowledgeable friend (or reddit/hn) to tell them what to use and then use it and trust that they know what they are talking about.
I'm pretty confident that as long as this page: https://matrix.org/clients/ looks like that, matrix will never see widespread adoption and it will never be the obvious choice except among the people who prefer to move their documents around with rsync and know what IRC is.
Matrix being a brand for a protocol rather than a full chat app is another self harm. The customer for a protocol is software engineers. The customer for a chat app is all humans with a phone.