I think many people overestimate the amount of Bitcoins that are actually lost forever. Many people don't want to sell them and there is also no point in sending them around. Also there is a big security concern to take them out of cold storage. So to me it is normal that many coins are not touched for 5+ years.
The estimates are probably about right, In 5+ years people tends to move at least a little bit for whatever reason. I do "have" some coins lost forever in the blockchain myself.