Open source software is a sustainable business in that you use it to foster a consulting business. Write something that fills a commercial need but is complex to configure, open source it, and then sell tech support to the users that tried to set up your system, sunk costs into it, then realised they were out of their depth.

Open hardware works similarly in that you offer your design and then sell a reference build of it. There will always be some percentage of users who are attracted by the fact that it's 'free' but just want the finished product and don't care about building it themselves.

It's a viable business model, it just doesn't scale the way closed-source scales. It only works when you're selling something new that hasn't been mega-mass-produced and commoditised yet. If you want to design something once then churn out millions of units, don't open-source your design.

It seems to be, at best, something you try and use as a value add to your real service like DB consulting.