People don’t audit anything, and pretending that they do is hopeful at best, deceitful at worst.
Even if you audit it you are likely not understanding the code well enough to figure out if it is vulnerable.
Which leads back to my original point which thus still stands; there’s no smart way to choose non-vulnerable plugins. One can obviously avoid things that don’t meet certain standards (popularity, lines of code, known issues, how they’re resolved, etc.), but still doesn’t guarantee anything.
This means that your statement about “smart Wordpress sites don’t pick vulnerable plugins” is frivolous. May I suggest “smart Wordpress sites chooses plugins carefully and limits the amount to those strictly necessary, but should still pay attention to updates patching issues”. Because that’s the difference between smart and dumb. Dumb sites are just left running whatever they shipped with, PHP or not, and smart devs make sure to keep their system and/or CMS and plugins up date.
And if you still want to argue that people actually review the code they depend upon I have one word for you: Heartbleed.