1. Browsers are often preinstalled for the user (especially but not only, on mobile Oses). Also, many browsers auto-update, so the new features will be automatically available eventually, without the user installing anything
2. The point is that while browsers are binary, they can then run a huge set of portable apps. As opposed to all those apps not being portable.
3. Everything Mozilla does is open source, and not just Mozilla but also most browsers today are open source - Chromium and WebKit in particular. This is a very open space.
"This is a very open space" - then why is it locked to one legacy scripting language of some sort of crappy transpilation they themselves don't use? It is effectively closed for new languages.
First, adding more options takes work. People need to volunteer to do that work, and prove that adding more VMs to the web can be effective (there are many technical challenges, like cross-VM garbage collection, sandboxing issues, etc.). People simply haven't shown this is practical yet.
But, people have meanwhile shown that cross-compiling to JS is practical, from things like CoffeeScript to C++. This is opening up the space to new languages, but it takes time and effort as well - again, the speed depends on how many people volunteer to help out.
2. The point is that while browsers are binary, they can then run a huge set of portable apps. As opposed to all those apps not being portable.
3. Everything Mozilla does is open source, and not just Mozilla but also most browsers today are open source - Chromium and WebKit in particular. This is a very open space.