I think people may be traumatised by what happened to Perl or Cobol. They don't want to become obsolete. In reality what happened to Perl isn't the norm imo.
PHP is the more recent one. Folks who worked in PHP but got out at the right time look at all the low-paying PHP jobs (and there are lots of them!) and see mostly-PHP-experience job candidates dismissed out of hand for higher-paying jobs that would offer work experience outside PHP and think "there but for the grace of God, go I".
PHP has declined but its by no means a dead or dying tech. Pay should be good in the good companies (Slack? MessageBird? I am sure there are some big names I dont follow the PHP world that much).
What I am saying is that even declining tech can provide stable income for decades.
Perl is a different story though. I dont know if the Perl people can still get jobs writing Perl.
Oh no, it's not dying—there's a ton of PHP work out there—it's just that most of the PHP dev market is very stagnant, wage-wise, outside a handful of companies, in a way that most languages in wide use are not. There's also a real stigma that goes along with still being mostly a PHP person these days, I've noticed. (I was one, like... 9 or 10 years ago)