awesome-paid-open-source

There are successful companies that are open source and seem to be doing well (though both have free tiers). Two that come to mind immediately are writing platform bibisco and email service Proton Mail. There's a list at https://github.com/mrjoelkemp/awesome-paid-open-source which isn't exhaustive, and, again, all have a free tier. So it's possible to do so but they generally use a free tier. Sorry if this wasn't helpful. I started this trying to figure out if I could find one similar (open source, paid only) that was successful, but negative results are important also! That said, your concern that someone might copy the code doesn't seem to be that big - I mean, no one seems to have done that with Proton Mail, which boasts c.100 million (!) users.