We (the Roots team) have had this same opinion for years now. I feel that WordPress underestimates the power they have.
Now WordPress does actually work with a lot of the top hosting companies to get their PHP versions upgraded. They do this quietly but they do talk about it from time to time. But they don't go nearly far enough with this outreach program and by still allowing 5.2 and not setting any deprecation timelines it really hurts their efforts.
Interesting to know, but... man... they've got some opportunity with PHP hitting a 7.0 and there not being a WP 5.0 released yet.
Upgrading PHP isn't just about using "new features" - WP doesn't even have to use any new stuff, but still require higher versions for security and speed benefits would help move the web forward a lot.
Now WordPress does actually work with a lot of the top hosting companies to get their PHP versions upgraded. They do this quietly but they do talk about it from time to time. But they don't go nearly far enough with this outreach program and by still allowing 5.2 and not setting any deprecation timelines it really hurts their efforts.