Edit: I'm not necessarily disagreeing with you but I just want to know what your reasons would be to see it die rather than be improved.
Or indeed developing a successor. Loads of people bitch about how bad PHP is - I see very few efforts to even come up with a better alternative.
Instead of the developers growing and improving, and so pulling the entire community upwards, many simply choose to leave. I've had this personally with Drupal: many, many years did I poor a (joined!) effort into getting proper TDD, OOP, deployment strategies into this CMS, but in the end I gave up. I mean: why put effort into making Drupal something it does not want to be; while a few weeks of learning a new language, the thing I wanted Drupal to become, already exists.
I think PHP is mostly a station in the career of many webdeveloper, before they move on to what are often considered "more professional environments". Sure, there are a few brilliant and very professional PHP-devs who stick around. But relatively seen, they are a tiny minority.
I have written good amounts of PHP code since then, and maintained even more of it. I've also used MVC frameworks like Symfony2, where few of the usual arguments against PHP apply, but it still feels like a low quality language with the tens to hunddeds of pitfalls one has to watch out for. And don't get me started on the Wordpress development that seems to be the main technology for webdev jobs today.
The argument that PHP is easy to get started with for beginners is a lie. Sure, you can install LAMP and start writing echo statements and it just works, but what then? The behavior of your code is sometimes weird, functions have cryptic names, and there's really nothing to point you in the right directions. There's no enforced or even encouraged conventions, and there's heaps of "Common Knowledge" one must acquire to write safe code.
You're not going to convince good php developers to stop using php, but you will convince the bad ones... actually go on with your bad self, ruby/python needs more bad devs
One is the fact that because of the implementation of PHP, where I assume a totally new process is created for every web request, a PHP app can actually edit itself and emerge different for the next incoming request.
Because of this, Wordpress has amazing features where non technical users can install plugins and even edit the source code of Wordpress, in Wordpress... This would not be as easily done in any of the various reputable languages I'm aware of... at least not in the standard implementations of those languages.
As someone who has developed wordpress themes, the plugin support in wordpress is a gigantic source of frustration. It is not amazing under the hood.