The 747-8I (passenger) and 747-8F (freighter) versions are still technically in production, but are currently being produced at a rate of 1/2 of an aircraft per month. At that snail's pace, Boeing will exhaust it's backlog of orders in 2019.
One of the last 747-8s to roll off the assembly line, if not the very last, will be replacement aircraft for Air Force One.
Correct. It's likely the last two 747's ever built will be the replacement for the VC-25A that have served the USAF since 1989.
What will replace the 747? Mostly likely, 777-8 and 777-9. The 777-8 will likely spawn a 777-8F freighter version.