Looking at the WordPress statistics, about 60% of WordPress websites runs on 7.x. I can imagine for dedicated servers that run PHP frameworks, that percentage is higher.
In May 2018 78% of Composer users were on PHP 7+. (Composer is the PHP package manager.) Source: https://seld.be/notes/php-versions-stats-2018-1-edition
As long as there's legacy, PHP jobs are not going to run out for sure, that's a given. I wouldn't exactly mind carrying a project in PHP >= 7, but I wouldn't wish to get a job in PHP < 7 territory personally.