It would all be the same ecosystem, just like PS4 and PS4 Pro. Whether you buy a PS5, a PS5 Phone, or a PS5 Portable, it would all be the same platform. You buy the game from PlayStation Store, and it is playable across any PS5 device. Developers build the game from the ground up to be scalable across all PS5 products.
At the end of the day, a PlayStation sold would be a PlayStation sold. More form factors means more ways for people to access the platform/software/service, meaning people can have both Home and Mobile, just Home, or Just Portable. It allows Sony to reach a larger audeince, and be even more engaged with its core consumers.
But from a financial stand point sony would still have to produce two hardware design which will cost them more if only just one of those system is popular and for them to not loose money the sale ratio needs to be 1:1, they would have to differentiate between those two devices beyond just grapical fidelity, In my opinion to validate the PS5 home console purchase Sony can tie it to VR, as in next PSVR headset will only be compatible on home console not on any portable device, but it will be a bold move.
One eco system sure, but consider this PS4 sold 80m and its just one sku, so just one hardware development cost. Now imagine this 80 mil PS5 but the development cost will be of two different hardware, so in order for sony to earn the profitability of PS4, Sony would need to sell PS5 and PS5 portable almost twice, almost at the level of PS2.
That is not how they run the PlayStation business. PlayStation Hardware is not the money maker. They sell consoles at break even or a slight loss. The profits for SIE come from Game Sales, and Network Services.
I don't think they have to do anything additional to give them their own identity. The Home console is for those that want 4K/60, UHD Blu-ray, and Next Gen VR. The Hybrid is for people that are fine with 1440p/30 docked and 1080p on the go, and also have an Android or iOS device as well. The Phone would be for people that just want one mobile device that can deliver their games, while also keeping them connected to their priority needs. There is a place for all of them, and if Sony has a unified architecture across all of the devices it is very easy to support, and R&D costs are extremely low.
PS4 is not just one sku. PS4 has actually had 3 very different Sku's, with 7 revisions in total. PS4, PS4 Slim, and PS4 Pro are all different devices that had their own R&D. They share core CPU/GPU technology which, would be the same case with the PS5, PS5 Hybrid, and PS5 Phone. With a massively scalable architecture like Ryzen and Navi, it wil be possible for Sony to have all three devices. Thanks to the focus on scalability across game engines, it wil be very easy for devs to build games that support all 3 devices.
This business model would be massively simplified over previous gens. Just look at last gen to this gen. Sony was supporting;
PlayStation 3 - PowerPC CPU, Nvidia GPU, and PS3 OS
PlayStation Vita - ARM CPU, PowerVR GPU, and PS Vita OS
PlayStation 4/ Pro - X86 CPU, AMD GPU, PS4 OS
SIE was supporting 3 vastly different platforms, with their own CPU, GPU, and OS. They had to build everything for all three devices from the gound up, whether it be OS related, Network related, or Software related. They where also working with numerous suppliers.
The purposed scenario for next gen would be;
PlayStation 4/ Pro -X86 Jaguar, AMD GPU GNC2, PlayStation OS
PlayStation 5, Hybrid, Phone - X86 Ryzen, AMD GPU GNC6, PlayStation OS
This is much simpler, much cheaper, and anything they develop is functional across all devices. Not only would they be able to support multiple versions of PS5, but thanks to PS4 using X86/AMD, they can continue to support it for years at a very low cost. With more options for entering the PlayStation ecosystem, they should only see an increase in Units Sold, and Total active userbase.