Vestigial is the right word; a characteristic of a surviving species that isn't particularly relevant to the question of why they survive. While debate goes back and forth between the various potentially-vestigial organs we may possess, it seems clear that if the appendix contributes anything to humanity's survival, it isn't very much. Also, vestigial tends to imply it isn't actively harmful, which I don't think it is because PHP certainly works just fine with other models as well. (If it were only page-as-program, I'd have another opinion... but it would probably also not be doing anywhere near as well on very large sites and we probably wouldn't still be talking about it as anything but a historical language.)

You may disagree that my characterization is correct, but it is certainly the characterization I intended to convey. That's why I parenthetically commented that I think the real reason it has carried on is another one, to contrast the relevant characteristics to the irrelevant ones. Creating a new "PHP-killer" and focusing too much on the vestigial features of PHP is a recipe for failure.