Otherwise we'd be seeing or would have seen a slew of 747-400 conversions which have had flat interest in the last 5 or more years, despite the 747-8F being available too - and instead, orders for the latter remain firmly intact.
The last 5 years we where in an economic boom. On top of that many A380 were delayed, forcing airlines to keep 747-400 longer in service then they planned to. Thats why we saw 747-400 conversions being postphoned.