And you can kind of flip this issue on it's head and ask - "Should PHP be releasing so many new versions so rapidly?"
As @SpacePhoenixAdvisor mentions, people will hold onto their old code for as long as possible. Perhaps this should be a message to PHP to slow down their development.
From what I see people value stability over cutting edge. "My site works just fine now, why update?"
Yet, when you look at PHP's development (and to a lesser degree, WordPress, Joomla!, cPanel, etc) they release updates like some lit a fire under their butts. PHP 5.6 is still in life, yet PHP 7.0 exists, PHP 7.1 exists, and PHP 7.2 is in release candidate. Do we really need that many different versions of PHP?
Because of this setup, I suspect PHP 7.0 will be skipped as a widely accepted default version of PHP. People will go from PHP 5.6 to PHP 7.1 (or maybe PHP 7.2 if this development trend continues). So that begs the question, what was the point of releasing PHP 7.0?
There is definitely a vast performance increase from PHP 5.6 to PHP 7.1 (I skipped PHP 7.0). We haven't yet switch to PHP 7.1 by default on our server yet, but it is being planned. I definitely understand the need for improvements in PHP, but sometimes I wonder if PHP is just releasing new versions just to be releasing new versions.
Edit: I should add that PHP voted to extend the life of PHP 5.6, that's why it's technically being supported longer than PHP 7.0 is. I think that was a step in the right direction, but the development of PHP still needs to slow down. Eventually it needs to become more like C and Perl and be anointed as "mature as it's ever going to be". That doesn't mean that updates don't flow out, but they become less and less.