You're right. WordPress forcing a tangible bump to the speed of websites is a significant incentive.

Looked from another perspective, how many game developers write their software targeting DX9 on Windows XP SP2 or Mac OS X 10.4 Tiger?

There have been many advances introduced to web development since 5.2's release in 2006. A developer of plugins for general use has to ignore the version restriction or 12 years of innovation. 12 years is a LONG time of doing things the old fashioned way. It makes features like name spaces, that were added to PHP way back in 2014, seem new hat. "4 years old" is not new in any shape or form.