Yeah, the traditional distinction between professors and degree holders who aren't professors wasn't part of the guild-training system, but probably better maps onto an idea like patronage. Someone who completed the doctorate was "done" with the training process, a certified master fully licensed to hold lectures and supervise theses. But this did not in itself come with any salary or position. The exact situation varied by country, but in Germany such a person might be a Privatdozent, someone who earned a living by giving lectures (often traveling from university to university) and tutoring students. That's somewhat analogous to a master painter who subsequently earned a living from portrait commissions.
I think the common way people view the situation has changed in part because these craft occupations are more difficult to sustain, so you're not really seen as "done" until you've landed a regular full-time job. Like being a painter without a salary, being a lecturer without a salary is nowadays a very precarious position. There used to be enough wealthy patrons paying enough in portrait commissions and tutoring fees for such careers to be make for a decent middle-class living, but it's not a common career anymore, so now people typically either look for a full-time academic position, or look for a full-time job elsewhere. You can temporarily hang around academia as an adjunct lecturer, but it's not a good or stable living.
A professor then is simply a master who, rather than earning a living as a craftsman, has a position and salary paid by a patron or institution. One might compare to a painter who became a salaried court painter, or one who received regular stipends from the Académie de peinture et de sculpture, rather than earning an income from seeking commissions or giving lessons.
I think the common way people view the situation has changed in part because these craft occupations are more difficult to sustain, so you're not really seen as "done" until you've landed a regular full-time job. Like being a painter without a salary, being a lecturer without a salary is nowadays a very precarious position. There used to be enough wealthy patrons paying enough in portrait commissions and tutoring fees for such careers to be make for a decent middle-class living, but it's not a common career anymore, so now people typically either look for a full-time academic position, or look for a full-time job elsewhere. You can temporarily hang around academia as an adjunct lecturer, but it's not a good or stable living.