This is an unknown number. Someone may have forgotten their password or remembered their password again someday in the future. There is no data to consider.
True and it could very well be that where the password was stored have been located by the owner and now s/he can re-access their wallets again. I have read in this Forum where some people will say that that have never moved their bitcoin since they bought them, all this can also lead to miscalculation of what you intend to know. So it might not be that because it is up to 6 or more years that some Bitcoin wallet have been idle means that those Bitcoin have been lost.