By rearranging some controls, and especially considering how much space the mixer section and two EFX banks take (and waste) compared to the similar-specked and much more compact SC Live 4 Mixer, you can easily increase the jogs by another 0.5-1 inch and keep the overall unit size very similar. Also, Pioneer AiOs (DDJ-1000, XZ) can have even bigger 8" jogs and remain much cheaper (and in the case of DDJ-1000 also much more compact) than separate units, so your argumentation fails here.
Tech designers need to improve space efficiency. It’s perfectly possible. Make the channel strips a bit smaller with the EFX section included, eliminate the rather niche grid edit section to move the pitch slider down, utilize the wasted space above the jogs, and voilà.
I agree with the “smaller, less features and cheaper” aspect, and there still would be enough differentiation points compared to a full SC6000/X1850 set, like even bigger and better jogs, deeper EFX control, more spacious controls, 2 separate screens, higher quality buttons/faders and the overall better modularity + failsafe aspect (if one deck fails, you can still operate your other one + the mixer, and swap it out later).
Everyone who prefers or needs to get 2-4 separate decks + mixer, which includes all major venues, will stick with those, no matter how badass the update for the Prime 4 successor gonna be like.
So yeah, here again my Prime 4 “next gen” wishes/ideas 
- More sleek, less green/silver flashy design language
- Slightly bigger jogs (7" / 17.7cm) with better jog displays
- More compact and “club standard” layout mixer
- Beat EFX as part of the mixer, but with own OLED screen
- Robust Start/Cue buttons and line faders + easier to service
- Keep the screen at 10", but make it stick out less + brighter
- Add Sampler as Pad Mode, maybe also Pitch Play (2 slots are still free)
- Better CPU, 2x RAM - they share these components with the AKAIs btw
- No more hard reboot needed to change into computer mode
- Bigger Auto loop encoder knob
- Separate and bigger buttons for: Menu, View and Lighting
- Mic and external sources can be routed to the EFX engine
- Master EQ makes more sense than Booth EQ imho
This concerns the hardware only, of course.