P: Add PSB as a file format for external editing

I think you're right about the two groups, but very few people ever set out to create PSBs - they do so only when they can no longer save their work normally. If you need PSB, you just choose it in Photoshop's save dialog and wouldn't need or anticipate making such a choice in LR.

There are two general use-cases for needing to use PSBs:

- Starting with a high-resolution image, well under LR's pixel limits, and adding many layers in Photoshop to implement a non-destructive workflow. 

- Starting with a ultra-high-resolution image exceeding LR's pixel limits, produced by a panorama merge or a specialized (very expensive) camera.

I believe that the great majority of LR users of PSBs fall into the first camp.  My belief is based on monitoring this forum and the community forum for a decade and on feedback from customers of my PSB Quick Look plugin for Mac OS.

A typical example of the first use case: The retouchers in an ad agency start with a full-frame DSLR photo of a running shoe and, responding to requests from their art director, end up with more than two dozen layers.

I don't think Adobe needs to spend valuable development time to support files greater than 65K pixels or 512MP in Lightroom.   I am sure the percent of people that need this is very close to zero.  

It's sufficient for the vast majority of the small minority who are forced to save as PSB.

so whats the point of that

we need more 

Lightroom Support for PSB is currently limited to files whose long edge does not exceed 65K pixels or the entire image is less than 512 Megapixels and it must be saved in Max Compatibility. Photoshop is capable of exceeding these values. 

It may well support PSB but you try importing them a modest 4.5 gb comes back with a too big error message

Or ask Adobe to remove any need for PSB?

Thanks for your valuable input John. Will pass this on to the team.