I think this is a massive downside for Linux, and saying "use unstable" just puts the onus on the user to solve it for themselves. Stuff like PHP, MySQL/MariaDB, Postgres and Python isn't part of the OS anywhere else, but it is on Linux. Making tons of third-party userspace software part of the OS distribution causes the distribution of that software to lag behind on Linux. I don't see any reason that what version of Debian you use should lock you into a default version of Erlang, instead of simply letting a user install whatever version of Erlang they need quickly and easily. For all the bleating during the systemd flame war about how Debian and Linux is about choice, it's a very constrained form of choice.
EDIT: Well, that's not true, I see the reason -- Linux isn't an OS so much as it is a family of OSes that are assembled from a lot of independently developed projects and are loosely compatible with each other, so the reason you need a Debian-specific version of Erlang is because Debian isn't quite the same as Arch or Fedora or what have you. I just don't think that it's such a great reason that users should just accept that it'll never get better.
I think you are looking at it wrong. It is not just "use unstable", it is "use unstable if you want to apt-get install stuff".
You can always install the version you want, you just have to compile it, including it's dependencies if needed (using a custom path for libraries).
If you need to do that for multiple machines, build your own packages.
You have a choice how you want to get stuff on your system.
People creating software packages that are in the official repository make it easy for multiple projects to reuse stuff and it works out for most people most of the time.
EDIT: Well, that's not true, I see the reason -- Linux isn't an OS so much as it is a family of OSes that are assembled from a lot of independently developed projects and are loosely compatible with each other, so the reason you need a Debian-specific version of Erlang is because Debian isn't quite the same as Arch or Fedora or what have you. I just don't think that it's such a great reason that users should just accept that it'll never get better.