1. Development is faster if your compiler (or code loader), tasks, and everything else is using all cores by default.
2. You get concurrent testing out-of-the-box that can multiplex on both CPU and IO resources (important given how frequently folks complain about slow suites).
3. The ability to leverage concurrency in production often means less operational complexity. For example, you say you can offload complex documents rendering to a background tool. In Elixir this isn't necessarily a concern because there are no worries about "blocking the main thread". If you compare Phoenix Channels with Action Cable, in Action Cable you must avoid blocking the channel, so incoming messages are pushed to background workers which then pick it up and broadcast. This adds indirection and operational complexity. In Phoenix you just do the work from the channel. Even if you have a small app, it is fewer pieces and less to keep in your head.
At the end of the day, you may still think the bullet points from the previous reply are not sufficient, but I think they are worth digging a bit deeper (although I'm obviously biased). :)