I don't think it was a case of "let's make the next greatest CMS... what language should we choose..?" I suspect it was more like "what can we build to showcas

I don't think it was a case of "let's make the next greatest CMS... what language should we choose..?" I suspect it was more like "what can we build to showcase Symfony." So, using PHP is somewhat the point of the project. But, considering your question more generically as far as if you wanted to write a Wordpress killer must it be written in PHP? I would say that currently the answer is yes. PHP hosting is ubiquitous and you would face an uphill battle if you also had to wait for shared host providers to provide another environment. It's kinda chicken/egg thing. If customers start requesting Erlang being installed on shared hosting then providers will start offering it. The thing is, most Wordpress customers don't give two shits about what language their CMS is written in. I'd argue that many of them don't even know what language it is written in.