Architects have copyright to their work. They do sell the same plans over and over. No one can just copy their work without paying royalties or at least getting permission.
Or, in other words - the default state of things is that copyright does not exist, not that it does. It's on copyright proponents to prove that we have a better world with it than without.
Great software developers already largely make their fortune doing bespoke work for clients with a need for it. So do great lawyers, great doctors, great system administrators, great technical writers, and so on. This isn't a new idea.
Or even better, grab some other people with knowledge of the domain and go find capital so you can do what the other businesses are doing, but better, because you're backed by deep knowledge of the software that runs your company and the other companies have no clue and no money.