It took Apple three years to bring Touch ID to the MacBook Pro (2013-2016). The iMac still doesn't have it. So at this point, who knows when Face ID will make an appearance on iMacs - could be next year, could be 2020...
the next question would be how often the imac pro will get refreshed.
as often as the mac pro? the mac mini?
will it be a one-time thing like the iphone 5C and SE?