That's honestly not really much of a "pessimistic" view, simply just a realistic one. Honestly I'd even bump that number to probably 80%. It's really safe to assume that people leave their funds on the exchange giants like Coinbase and Binance simply due to the ease-of-use and convenience factor.
Personally, it bothers me when someone does something without really being familiar with it - if nothing else, at least the basics like secure backup and accepting the idea of "not your keys, not your coins". I wrote 50% because I want to believe that at least half do it somewhat correctly, so while each lost coin only increases the value of the others (Satoshi Nakamoto himself wrote it) I’m not a person who rejoices when someone does something stupid and loses any amount of coins forever.
If at least some people found this forum before investing in BTC, I believe that most of them would learn a lot and avoid some difficult moments in their lives.