I think the theory is that the payoff comes later. Ideally the system quietly continues working well for decades, but needs maintenance and improvements 30 year

I think the theory is that the payoff comes later. Ideally the system quietly continues working well for decades, but needs maintenance and improvements 30 years from now, by which time most of the existing C++ programmers have retired. If this hasn't happened for PHP, maybe because there are fewer critical infra projects written in it. If it's just another web service, that can be rewritten (unless it's at the scale of Facebook). Whereas the tens of thousands of firmware code bases and other critical infra - we won't even think about them for decades as long as they continue to work (or appear to). Let's check back in 2038!