It doesn't seem impossible in principle, but anything viable has to be better than any open source tools that might be out there but also able to command an audience with enough money and inclination to pay for it despite whatever level of license infringement takes place. It sounds like quite a narrow scope!
(yes the domain name is silly, rebranding is on the todo list but customers come first).
It has plenty of customers paying a $45/month subscription, some of which are big name companies that unfortunately don't like to have their logo put on the web page. But it's a CLI tool, and it's monetized. What do you want to know?
For better or worse there are enough users and customers (it's free for open source devs), that fixing problems and addressing feature requests/support tickets tends to suck up the available time. I still live in hope that one day I will drain the high priority queue enough to be able to reorganize the docs and redo the web presence.
Conveyor has very small server footprint and I use a rented Hetzner machine as the primary server for everything (there are backup replicas for the licensing servers). Customers pay for the CPU load, disk storage costs etc because it runs locally. So the ongoing baseline costs are very low. There's also no costs involved with handling customer data, which can be significant for a SaaS, much lower demands on the security and enterprise compliance side because we don't have any sensitive access and so on.
It's definitely a niche product area. I didn't pick it because I thought it'd make me rich. It was a sort of retirement/burnout project, I was tired of doing projects in organizations or groups that ended up killing themselves through political activism and backstabbing. I was fortunate enough to be able to self fund the company until the product was mature, without needing a VC raise, so I was able to do things exactly as I wanted to. The code is clean, the customers are great: they are people just like me so we understand each other very well.
The nice thing about Conveyor is that it's such an unfashionable and small market, there's almost no competition and isn't going to be any. It doesn't yet replace a tech salary but it grows steadily as a business, and I don't have to worry about some VC-pumped startup giving away my product for free for a decade, killing me and later discovering they don't know how to monetize themselves.
In my space, some larger competitors are more focused on SaaS/cloud offerings. But that can be a mixed bag for security-sensitive customers who prefer to self-host, as well as for folks who want to integrate a tool into an automation/CI/CD pipeline. CLI tools are more compelling in those situations.
If I was to start something like this today, personally I wouldn't use a permissive license for the free version next time. That's been really problematic as a bootstrapped business.
Fun part is that the CLI handles all the browser redirecting for the payment and saving the license (I guess). Just pass it a `--pro` arg and there you go.
But I’ve seen REPL instances (which are command line in a way but interactive) monetized. The AMPL optimization language is monetized. The very expensive KDb+ language and database is monetized.
Back in the day lots of DOS shareware tools like pkzip, etc were monetized as shareware.
Bought it to transfer mail from SMTP to Office365. Worth every penny.