While what Kaguya says reeks of Hipsterness, she likely saw some of her favorite doujin series die because the artist went pro before they finished it.

Kaguya said:

And there are a lot of people that stopped drawing either because they quit or went pro.

Well, Sou made doujins from 2005-2009. In 2011 Hanayamata is serialized and in 2015 we got Ochikobore Fruit Tart. They only make 2 more doujins after going pro.