Only had ten years to move...
The SAP Movement opportunity has been available for a long time, with ever improving tools to enable customer success.
The move from on premise to cloud infrastructure has been the low hanging fruit to move from capital expenditure on premises to operational expenditure in the cloud. However this just provides financial benefit as ECC remains. The infra move to cloud can deliver an opportunity to start with HANA, so ditch database licence costs and do some limited code improvement. But the bigger benefits are in S/4.
To get a transformation to S/4HANA requires migration of your distinctive enhancements or a fit to standard approach to deal with long term cost of ownership challenges.
The UX improvement opportunity can be started on ECC Fiori but that transformation is not complete so still requires Enterprise Portal to bring it all together.
S/4 provides Fiori UX (UI5) as the foundation and the ability to keep the core clean while side by side extension on Business Technology Platform (BTP) integrates with a single design language.
S/4HANA private cloud edition (PCE) provides both infra and application cloud platform benefits with the ability to integrate SAP and 3rd party services in a scalable hybrid architecture. Key for reluctant DSAG users is the ability to use familiar ABAP extensions (RAP) or modernise to supported languages or bring your own language (CAP).. Test automation and CI/CD help to keep cost of ownership under control within the Cloud ALM framework.
Extended support may buy a little time to smooth the human resource limitations but well supported applications are a real business requirement and the innovation by SAP the competitive advantage icing on the cake. If you snooze, you loose..