probably the bitcoins that didn't move in the next 20-30 years top can be considered lost forever.
No, Bitcoin was created like 12 year, in 20 to 30 years to come, bitcoin will be around 42 years old. Did you think some people can not hold for their children and which can be more years. Also all lost bitcoin may not be known in total because as time goes by, some people will also lost more bitcoin.I really don't see a single reason why someone shouldn't move a satoshi after 30 years, especially because he would be at least 50-60 year old at that point. Hodling till death is kinda extreme 
It can be caue of many reasons m, inheritance purpose is one of the reasons.