I'd say the most likely reason would be the modularity of the user land. Namely it makes the adoption of such backwards incompatible changes much easier. Anyon

I'd say the most likely reason would be the modularity of the user land. Namely it makes the adoption of such backwards incompatible changes much easier. Anyone can make some new piece of software to do something in a new way(ex. init systems). And there will be people who want to use it and don't want to use it. Most of the time distors (and forks) will arise that do it the new way and people who want that will pick those distros. This is really common in the init-systems vs systemd debate.