TBH, PHP is viable for pretty much exactly as long as Wordpress survives and uses it. If that changed, I wouldn't fancy PHPs chances.
I do concede that if PHP went away, WP would somehow survive, like some kind of post-nuclear cockroach.
The performance characteristics of Node are completely irrelevant for most sites. The developments model's inherent complexity and the moving target, however, matters very much.
The vast majority of blogs that are build in Wordpress. Remove all the blogs and corporate websites and I bet that's not the case anymore.
And no, there's some pretty big, modern sites like Mailchimp which run on PHP.
There's literally no reason to use Node if you already have an effective backend with PHP because Node doesn't really bring anything new on the table, but it does bring along its own class of issues.
You can't write a production grade web socket server in PHP. and no ReactPHP isn't my definition of production grade. If your app is real time you just can't use PHP for that.
So yes, Node brings something new to the table.
You just said, Node brought nothing new to the table, obviously you were wrong.