Boeing still foresees a viable future for the 747-8F - there are a whole lotta 747 Freighters out there that are getting really long in the tooth and will need to be replaced with something (and there are not many passenger 747-400s left out there that have enough life left to make viable freighter conversions)
The 777F is proving to be quite popular, but the 747-8F can carry ~ 40% more payload, and the nose door gives greater flexibility for oversize loads.
Right now the Boeing focus is to keep the 747 line alive until the freighter market picks up enough to drive significant new orders.
A replacement or follow on E-4 could be based on a -8F airframe, and I think Boeing hopes to keep the 747 line going until at least 2030 (at an admittedly low rate - supposedly they can make it cost effective at one/month rate).
As for a new AF1 to replace the 747-8 based aircraft? That likely won't be until ~2050, I doubt any of our crystal balls are good enough to predict what passenger aircraft may be in production by mid century.