Language is a living thing. Unless we're talking about Ancient Latin or Greek, e.g. BUT... even ancient, dead, languages can be brought back to life. Look at Ivrit. It's ancient Hebrew brought back to live, and it's very different now from the Hebrew of the Scriptures.
Look at the English of Chaucer or Shakespeare - and then look at today's English, in all its variations.
I think of this old Dutch sentence, which I first heard when I was a young lad in school. "Hebban olla vogala nestas hagunnan hinase hic enda thu wat unbidan we nu." At first I couldn't make sense of it at all. Then the teacher started explaining and sure enough, it made sense. I could see how my own language had evolved over the centuries, and it was fascinating.
https://en.wikipedia.org/wiki/Hebban_olla_vogalaWhat I'm trying to say here is this: What once was correct isn't anymore, and what used to be wrong may over time become correct.
A professional writer will of course write properly, using correct words, spelling and grammar. Unless... oh, there's this area where a professional writer might have good reason to want to influence the way his language is evolving, and he might choose to introduce neologisms.
But I think what sets a professional writer apart most from amateur writers is not the fact that they use their language correctly and get paid for their work. I think it's their attitude that matters most.
I don't consider myself a professional writer, even though my writing is probably good enough. I even think I usually deal with critiques in a professional manner. But I don't want to be a professional because I want to write for me. I don't want others to have a say over what I write or when or how. And I don't want to have to deal with the administrative and monetary parts of it.