https://www.checkbot.io/ is a paid Chrome extension I built that automates many of the checks I used to do manually during contracts when optimising the SEO, speed and security of websites. I found many developers and content editors on websites I worked with weren't checking for web best practices because of lack of knowledge + time and because doing manual checks is error-prone + tedious.
I'm guessing it's extremely rare you can create + sell something with minimal hours invested with little specialised experience though.
I agree though, I think you're always going to have some level of ongoing responsibility as well e.g. responding to customers/clients, keeping an eye on server issues, checking up on contractors, marketing.