Python scales quite fine actually (Disqus and YouTube use it for a majority of core development). It's all about the tech behind the scenes (uWSGI, Tornado, Ngnix, memcache/redis, etc), not the language itself (just as it is with PHP). Try scaling PHP if only using Apache Prefork instead of mod worker + php-fpm or Ngnix + php-fpm and you'll see what I mean.
I have no problem with using either PHP or Python on a particular project, depending on what is involved. I wish PHP threads would show a bit more maturity/pragmatism on both sides of the aisle sometimes.