What exactly is your argument, that it's not important because people could use something else if they had to? Does anything qualify as "important" with that definition?
There seems to be some confusion about what "it" refers to. You keep referring to Safari. The original poster referred to KHTML, which for most intents and purposes "turned into" WebKit. That's how I and apparently most others understood him to mean.
WebKit, the rendering engine originally used in Safari (not Safari the application), nowadays powers Chrome, iOS, Android, BB and a hundred other browsers/platforms. In simple words: the absolute majority of mobile browsers use webkit, plus >50% of desktops/laptops.