How about a DP with built-in Pianotec and VST instruments?
How about custom made DPs? Select audio / BT in, speaker (size), 76-88, etc? Why "one size fits all"?
Got it as to economies of scale, but I have the impression people are over and underpaying for what they want.
About Pianoteq : you can purchase a Steingraeber piano with its Silent module made in collaboration with Modartt. I suppose that the Piano Phoenix by Adel H came from a collaboration with Modartt too (pure speculation).
About “any” VST instrument. A VST instruments is optimized to be controlled with a screen and a mouse. A digital piano has typically a very little screen not usable for most VST. A typical DP should not look like a workstation synthesiser. Then an adaptation of each virtual instrument is needed to use efficiently the available screen/button set.
A custom made DP does exists. It is the Viscount Legends’ 70s where you can choose the keyboard (3 options), and add different sound modules (Acoustic piano, Clavinet, External module) to the default E.Piano and Sound Collection modules.
Some synthesisers did have some extension possible (Motif ES with its PLG cards, Roland synthesisers with SRX cards), but now, the extension is mostly by downloading libraries.