We're not expecting to make our money back on this one. And these amounts are a drop in bucket compared to many other Google products anyway.
So a cynical profit motive is not why we're doing it. We're doing it for the stated reasons, to move security forward on the Web; see https://security.googleblog.com/2017/09/broadening-hsts-to-s... and https://security.googleblog.com/2018/02/a-secure-web-is-here...
Also, I could talk your ear off about the design of our infrastructure for hours. Suffice it to say, it's a lot harder than you're making it out to be, particularly as regards to scaling. Our registry platform is open source, so feel free to inspect the code at https://nomulus.foo . And that's not even getting into DNS hosting, which involves a very large number of instances distributed around the entire globe.