Indeed, the 789 is a bit small to replace all the 744s when there are options in the form of A351s, 777-9X or the A380s VS keep postponing that are more suitable 744 replacements.
I believe the 744 will still be flying in VS colours until the end of the decade at least, especially considering that VS refitted the LGW/MAN/GLA 744s with all-new interiors and IFE a year or two ago.