How do I help them resolve this?

I have recently started working at a new company, it's a very small company with only 1 scrum team.



We have a successful product, and we use a service to host that product.



We're trying to remove our dependency on the host, by building our own host service. 



The PO has used this service for many years, knows it very well, and wants to design this service to mirror the functionality of the one he's using. 



The lead designer, has worked on services for 20 years, but on ones that compete with the one the PO is using currently. 



I've helped them to resolve a lot of the terminology issues, but we're still having a major hang up. 



The lead developer wants to build the backlog based on architectural design, and is thinking bigger than just the current product we sell, but to future products other customers might want.

The PO, wants to design based on the existing product, and service.

I've tried customer journey mapping, and even several working sessions to try and establish a clear product goal, and a few features that would deliver a single slice of functionality.

But, the lead developer is concerned we'll design a solution that is short sighted, and doesn't enable future needs, and could lead to serious issues.

At the end of the day, the lead developer will give in to the PO, but it's a struggle trying to merge their needs. How would you go about helping them visualize a goal that delivers the needed functionality iteratively, but doesn't overlook the potential future needs?

Keep in mind, the "customer" is the PO, this is all being done behind closed doors. All of my training says deliver iterative, single slices across the stack... But, scrum assumes a complex problem without a known solution... So, I'm not sure how to help coach this.