api

My expectations are that you would need deep pockets for licensing and development, however, I don't know any figures. The API appears to have changed since Firefly, but again I know very little about specifics. 

https://developer.adobe.com/firefly-services/docs/photoshop/

I seem to recall that at some point using Photoshop on a server was explicitly forbidden, but I cannot find a pertinent section in the current General Terms of Use of Adobe. 

https://www.adobe.com/legal/terms.html#

So I am unsure about Adobe’s current position … 

Edit: At the time I assume the reasoning was that Adobe would not allow a license to be used to »rent out« Photoshop-services … 

Interesting question, Archie.

As a former programmer, I ask "Why"?

And further, calling an EXE or APP would require access to all the connected parts inside Photoshop. Things like fonts, plugins, settings, credentialing, file access rights, permissions (Read Only, Read&Write) etc.

Anyway, Archie, just musings. One of the Photoshop experts will be along shortly with  better set of questions.