WordPress is like the Kalashnikov of the web. You can use it for pretty much anything, run it anywhere, do god knows what to it and it'll still happy hum along (albeit behind a very big varnish cache). A few of our clients have millions of pageviews running off of a couple t2.small instances and the single biggest factor for them is the UI and the security of knowing they're not tied into a bespoke platform.
If everything is cached, sure, serving static content is cheap. The comparison with a solid machine I find it funny, a bad update in a plug-in can render your site useless for a while and WP's security record is abysmal.
I previously hated Wordpress because it was a source of so much trouble on the web, but now I sort of grudgingly appreciate it. I still don't enjoy having to work with it on occasion for clients, but it doesn't make me grind my teeth anymore.