Earlier quoted context omitted.
Were either of the first two started by "a sane engineer with a bit of experience"? Inertia is powerful.
Perfect is the enemy of good enough. Doesn't matter what your site is coded in if it isn't revenue positive/have funding/etc.
Though, to be honest, I don't think that using Play (today) or Rails/Django (yesterday) is sufficiently less productive to merit the technical debt you almost certainly will incur starting with a PHP stack. (And I am an ex-PHP person, to the point of hacking around in PHP core...that's a few months I'd love to have back.)