I hate to even appear like I'm defending Sapkowski, but there are some things that should be clarified:
Can't make rent off your precious classic of a fantasy novel alone? Boo fucking hoo.
Until not that long ago, authors in Poland were given some of the shittiest royalties possible. I would not be surprised in the slightest if he got ten or twenty(!) times as much money for selling the rights to the books to make the infamous TV series and movie (don't watch it, it's terrible but not even in that "so bad it's funny" way) than he got from royalties. This shit was barely regulated by law in the 1990s. Series ended around 1997 and he had to sell the rights in the early 00s.
Yeah, Sapkowski is a shithead and an alcoholic. I don't think that I have met anyone who enjoyed talking with the guy on any occasion, so there's that. But think of it like this: if an author of a series that most Poles aged 15 to 50 read or own does not have a lot of money, there was/is perhaps something wrong with our publishing.
And here is my belated response to it: for each their own. I wrote quite a few short stories myself and can tell one thing from experience: I'm a lot better when I'm not constrained by someone else's world. I've basically learned how to read and write with RPGs in the background, watching how my brother played Ultima and Baldur's Gate games. But I rarely like to both read and write in a fanfic format. The quality of fanfics aside, they don't feel right to me in the sense that the characters that I know (and possibly even like) suddenly act out of character. Usually, I can't think of a good way to add or expand on someone's creation. But I love to make my own worlds. Test various concepts in my head and write it down. If I was told to make a fanfic it would be a complete crap. That's assuming that I could even finish it.
Here's what I wrote so far and felt like it's good enough to be kept:
About four times as much stuff I just threw away or burned because it didn't meet my own standards or would need a revision so major that I might just as well start anew. Effect? One of those stories actually got published. Yeah, to this day I can't really believe it.
So what, does it mean that the only good way to learn how to write is to create your own world, characters and abhor the sheer idea of a fanfic? Fuck no! It's just something that I can't do. I also write everything in pencil because I can't seem to focus on writing when I'm in front of a keyboard. Does it mean that nothing of worth can be created on a computer? Again, fuck no! That's my own quirk. From my experience I would tell you to maybe turn the computer off and try writing your drafts on paper, but that's just that and nothing more.
As far as the writer's training goes, it's perhaps a good way for some people. Just like fanfics. Others are just as entitled to their own opinion and take on the creative works as you are. Going so far as to hate an author for his own experience with writing and fanfiction sounds like a knee-jerk reaction. Yes, perhaps just like theirs but it does not justify either.
You have a point in calling Sapkowski on his hypocrisy, but that's only assuming that he didn't grow as an author since the mid-1980's. I think that he did grow, although developed a nasty case of Cerebus Syndrome despite being imo better suited to write light parodies.
Seriously, for each their own. So they don't like fanfics. Fuck 'em, but they are absolutely entitled to having their opinions.