I strongly disagree with this analysis. Apple's strategy is not a law. If a third party thinks they can make some bucks by releasing something stackable they will. Apart from that, the Mini is still very often used in a server setup and there is a market for stackable peripherals there. It's only a matter of time because there is certainly demand for it.
It's true that stacking is not straight forward with this form factor, but with some cleverly designed "feet" it's certainly possible. Feet which had also been designed by 3d parties to make the previous generation better stackable btw...
An informative thread here:
https://forums.macrumors.com/threads/1024699/