Modernisation a customer demand.
Customers all have cost pressure to varying degrees so bear down on all suppliers to do more for less. The SAP maintenance fees are no exception.
So what drives that cost and can be controlled or influenced by sap?
Good design already reduced the difference between the mainstream databases or operating system however they still need testing to assure quality.
Test Automation enables productivity and lowers QA costs, but there's still no getting away from thorough testing. So reducing the number of database products and versions is a powerful way of reducing the test effort so maintenance costs for customers.
Reducing operating systems down to Posix would be a similar cost lever.
So ECC 6.o to S/4HANA is motivated by avoiding the maintenance cost of hundreds of database/operating system permutations as well as being a modernisation opportunity for in memory database.
Realistically customers will have been running upgrade projects on ECC and associated products over the years, with full scope testing efforts. Whilst S/4HANA is not an upgrade to ECC, the transition can be a similar effort to Upgrade, with the opportunity for Simplification for those focused on running better.
Bottom line is that Customers have demanded this, not SAP, because modernisation is in their benefit.