Rarely will a data migration take place in a vacuum. There are particular reasons why it needs to be done. Sometimes these are purely technical but, more often than not, there are business drivers.
The shift to SAP S/4HANA is a perfect example. Progressive organizations understand that making the switch now will save a lot of pain further down the line when SAP stops supporting older systems.
Yet it is not just about the availability of the S/4HANA upgrade. (You could always wait just a little longer to change systems.) It is actually about introducing more efficiency and productivity into operational workflows as soon as possible.