PHP did make it slow. They eventually wrote a tool to transpile PHP to C and then compile down to native.

Sure, PHP was obviously a very poor choice as an main language. Facebook still today suffers (i know people at facebook), and even pured in millions for HHVM and Hacklang.

If they had opted for C# they could probably have been able to run it as-is today.

Obviously. But a sane languge could/would just require better design and performance tuning. Not reinvent a new languge kind of change. At scale you need to design carefully no matter if its Java, C or Rust. But you mostly can keep the languge and not write you own.