Forgood, spoken by a true now developer who understands very little about the web and how sites are constructed.
WordPress is very popular, that is a given, it is a good platform for what it was intended to do, that too is a given. It is not by any means a "Best Practice" in web development and in no way does it make up 70% of the world wide web. Where you acquired that figure is beyond me, but it is not where even close to that.
Even if you use the very skewed figures of the WordPress development community, which are constantly challenged by the W3C and IETF, there figures are right around 20%, and many people in the know challenge that figure, there is really no way to know for certain how many sites are developed in WordPress.
A better question would be, "How Many Professional Websites are developed in WordPress", last time I checked, there were no professional eCommerce or corporate business sites developed in that framework, huh.... why is that..... something to think about.... of course Disney does have a blog system developed using WordPress but that site is not even linked to their corporate site, .... hmmmm .... could it be for security reasons....
Something to think about....