Legacy code is side effect of any successful company. The longer your grow, the older your code is.

How can we deal with it? Rewrite? Gradual refactoring? Hire external developer team with legacy skills?

In 2022 there are tools that can handle like composer updates your dependencies. Any company or developer can refactor their huge legacy code in a couple of weeks. Bump PHP 5.3 to PHP 8.1? From PHPUnit 4 to 10 or Symfony 2.8 to 6.1?

You don't have to know a thing about the BC breaks and how-to-chagen it.
I will show you, how you can become master of change with simple command line.