I would expect something like: popularity does not means good.
This refered to the fact that people like something because they think it is better for them, not because in fact it is.
Mostly PHP and Wordpress are good, but not been based on Zend framework is not the good part, it is the worst part (my opinion). This because there is nothing inaccessible in zend framework that would frighten people and make them leave wordpress, it is just a matter of not show that part (but beeing based on it anyway).
Another school of tought is that something is popular because something is popular. Just it, so there could be no relation between wordpress and php, zf or anything that is growing up or going down together, and this is because there are not enough data (there is only an (almost) growing line), and because the experiment is hard to reproduce (maybe we need another planet, wait some milion year, hoping there would be some culture accessible to our language .... ok, give up)
That said I almost agree on the article, very enlightening