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Team members struggling to thin slice business value - how to help?

I have been asked to join a new a team which is struggling with agile adoption within a large programme utilising fairly methodical Scrum. The aim of the product is to create a surface layer over the current legacy customer information systems (There are 4) which ties all of them together into one common interface and (eventually) replace the systems.

In the last few weeks we have embedded some of the key values, the necessary ceremonies and artefacts and begun to display real team cohesion and cross-skilling.

However the team is really failing in one key area.

They originally ordered their product vision as a full end to end process (no problem) then they boxed the various systems and sub-systems that a customer service representative must use to support a customer. They listed out the functionality that the customer service reps need (File Access, Information Update, Work Queues, Note Taking and 40+ others) and simply labelled these as epics.

Then they put the user journey on paper, dropped the epics onto the diagram like lego blocks over the user journey and start working through them. First part of the user journey, first epic, deliver that. Review. Next epic, deliver that.

For example

A user needs the ability to see a list of customers. Therefore we will recreate the pending customers as a queue of work. That is an Epic and will take a Sprint. That is our Sprint Goal. We did that.

A user needs to take notes. Therefore we will create note-taking functionality and call that an Epic. That will take a Sprint. That is our Sprint Goal. We did that.

Repeat for 40+ epics...

Understandably the stakeholders, well versed in Agile and Lean Product have intervened to say the team have not thin sliced the service. They have simply created a series of building blocks which are delivered in a spiral iteration...block after block and once all the blocks are in place the service will work end to end. Without the delivery of all blocks then the business cannot use the value...

A senior stakeholder asked the team just to demonstrate a message saying "Customer Name_EXAMPLE" could make it all the way through the system before adding in additional functionality.

The solution designer and the business analyst are struggling to understand thin slicing and have started resisting the change despite the Scrum Master and Product Owner being in agreement. They want each bit of the system laid like a paving slab before the next part is started.

What strategies and examples exist to coach individuals into effective thin slicing of business value?

Edit: If any clarity is required then I can edit but really looking for genuine guidance on this issue appears multi-faceted.