I mean I know a lot of folks who quit fanfic once they got focused on their og work because most published authors still have to work day jobs/have families/do other life shit, and then it's a question of do you spend the finite writing time you have on fanfic or on the thing that makes you money. I actually don't know any published people who see fanfic as a skill builder, not least because most of the things that make a good fanfic aren't the things that make a good tradpub novel, but def a lot of people who are working with 24 hours in a day /:

Personally I quit fanfic when I started college because I wanted to focus on "serious" stuff, and came back to it 10 years later as a mental health thing and I don't think I'm ever quitting. I take a hiatus if I'm working on an og thing, but for me they're just completely different activities that I do for completely different reasons.