fwiw, the team that created Matrix almost exclusively used Java serverside from 2003-2014 (when we switched to creating Matrix). The last gen of Java servers we wrote were super efficient and nice to maintain thanks to netty.
The only reason we switched to Python and Twisted for the first gen Matrix server (synapse) was for rapid prototyping using a platform that we reasoned the open source and selfhosting community would already have installed and be comfortable with. Java felt way too enterprisey and non-open-source-friendly, making quick tweaks to the codebase a huge pain, not to mention the verbosity of the language. The team agreed that expecting casual folks to install and use a JVM just for a chat server would be a major turn-off, and we continue to feel that was the right call.