Don't forget to weigh the culture driving a language's development
I would also weight the fact that development on python/php is actively done in public. You have one group of individuals nailing down a standard specification that is freely available to anybody/everybody.
Much like the W3C does with the HTML/CSS standard. You have a small group of motivated individuals who control the finer details of what the language is designed to accomplish. Everything goes into a clearly defined specification before it's released to the public.
OTOH, languages like LISP are forked behind closed doors by professors or other individuals that genuinely believe that their perspective on the 'best use' of the language is right. They may be simultaneously right and wrong at the same time because some implementations are great at certain things; while none are the best at everything.
That's not necessarily a bad thing because diversity breeds innovation. Languages like LISP are, and will remain great languages for learning and research because they push the boundaries of understanding.
But the qualities that make an environment good for innovation aren't necessarily beneficial for stability; conversely, the qualities that make an environment good for stability aren't necessarily good for creativity.
When development is based on active collaboration, sometimes individuals are forced to concede for the benefit of the greater whole. Bad for research/good for consistency.
The fact is, we're still living in the wild-west of programming language development. The problem of designing the 'ideal language' is so great that, despite monumental efforts, nobody has come close to solving it.
In the research/academia sector, there's still a lot of room for improvement and innovation. In the commercial sector, where there is an exponential growth of software being use in practical applications and the driving force is simplicity and consistency.
Some languages specialize in the former, some specialize in the latter. Those that try to specialize in both usually don't do either very well and die off.
By both, I'm referring to monolithic languages like VB/C#/Java. It's too early to say but I'd like to see what C# and Python look like in 10 years. At the current pace C# is growing functionality and inconsistency at a rate that makes it look pretty grim. Even with great documentation, it's just too much of a pain to remember all the subtle details and quirks included in the language. It's great for a single developer but as soon as you throw in more developers with unique styles, inconsistency in the codebase grows, quality suffers, and nobody wins. I think there's a lot to be learned from the difficulties Perl presents in a production environment.