I think the bigger argument is getting WordPress itself to move its version floor. As derivative works, Plugins should not mandate a higher version of PHP than the root work.

If you look at the WordPress' current Requirements page, they support all the way down to PHP v5.2.4

At 5.0, WordPress itself should simply cut off any an all PHP versions that have reached End-of-Life. And then, as each subsequent version of PHP reaches EoL status, cut them off in the next version released ~6 months later.

Plugin authors can maybe help push it along and increase end-user awareness by having a function that tests the PHP version and if it is less than 7.0, display a message in the Admin area to that effect. I believe the WordPress marketing team either has or has been working on a site section about the advantages of upgrading PHP versions, so it can point to that site.