Exactly this. If you already know PHP and can produce results (mostly CRUD apps) and it doesn't need millions of rps, why do we need to use another language because they are so much better ? So much better at what ? It is like saying "I have a hammer, so everything looks like a nail to me". Not every web app has to be written in Lua/Rust/Go/Julia etc. Each language comes at a cost. No free lunch.
"But why should anyone start a new project in php now that we have so many good alternatives?"
Because everything comes at a cost. You want to use Rust instead of PHP ? Sure, sounds great. Now lets go find a really good Rust developer who also understands how to host it correctly and then if he leaves, can I easily find another Rust developer ? Hmmm. You also need to account for maturity, ecosytem, hosting, maintenance etc which are all real world problems. Building a new shiny toy project ? Go ahead use whatever language you love. Again, it is about cost vs benefit.