I wonder if you're familiar with the Hack programming language (https://en.wikipedia.org/wiki/Hack_(programming_language)), which was developed by Facebook as an extension of PHP that addresses some of the issues that Facebook had with PHP as the company scaled from its beginnings as a social network for college students to a colossal network.

I personally believe implementing a multi-billion dollar enterprise on the foundation of PHP code isn't a mistake. Even if there's a lot of tech debt, there's a ton of money to pay off that debt, and then some. Most of us dream of having that type of problem.