I believed every website owners nowaday knew about the value of domain traffic.
99% of website owners will knew about google adsense and at least 90% of them knew about domain parking.
There are no reasons for them to drop domains with traffic and let the registrar make a profit from them.
In this case, the domain drop will narrow down to the reason of domain owner forgot to renew or the owner accidentally died. However, domain owner will have a better contingency plan from time to time to avoid such accidents from happening.
IMO, expired domain auction will eventually become a history when registrar unable to grab some high quality expired domains.