[agos]

how much of that ⅓ of the internet is running third party plugins and themes on their WP installation?

[Macha]

If they hired bespoke custom development from contractors at the price range these WP sites are developed for, do you think that would be better code?

[dawnerd]

How much of the rest of the web are using third party dependencies in their code?

We’ve built sites for a very large social media company where everything had to be audited before production including third party plugins. WP VIP has a list of plugins they’ve vetted and/or applied their own patches to secure.