I think they quit the anime jobs entirely and move to more marketable jobs like graphics design. This 90% claim is more or less anecdotal, as I read that the industry veterans say that they saw only 1 out of 10 young people survived in this industry. Many work environments are similar to each other: there's no written contract and they have almost no unemployment insurance, etc. I don't understand how those anime lovers turn blind eyes to this and still somehow "enjoy" anime. It's horrible, horrible stuff.

A detailed survey in Japanese: http://www.janica.jp/survey/survey2019Report.pdf