As the brochure about the extensibility of SAP S/4HANA has been published recently it is obvious for me that SAP is offering a wide range of choices for all partners for their own software of today and tomorrow.
Everybody has its own pace and priority when it comes to his own piece of software. Some are driven to rapidly innovate due to market demand and some can remain for a longer period. It depends on the application area, industry and country, educational and budget in the individual case.
In this situation the duo SAP S/4HANA and HCP play together as a team where each one has specific strengths.
The HCP is our first recommendation when you look for a development environment to start from scratch with limited upfront investment (edge applications). Additional use cases to that are if you want to
Reach out to external users and consumers, enable an external Fiori and Mobile user experience (e.g., customer service portal)
Enrich standard processes by before and after steps (e.g., customer service)
Manage choreography of several systems and data sources for new business processes (e.g., sensor data, IoT, Big Data)
Bridge applications between Cloud and On-Premise
The nature of these use cases is to be on the side of the business application SAP S/4HANA, so that we call it the Side-by-Side Extensibility.
If you are looking instead more in the direction of the following use cases like to
Enable an internal SAP Fiori user experience (desktop)
Create variants of standard processes and business logic (e.g., micro vertical solutions, localization)
Focus on company owned business documents (e.g., legal compliance and company policies)