They've invested heavily into new manufacturing processes since they designed the cheesegrater. Those processes can produce things like the garbage can easily enough, but they can't be made to produce the old cheesegrater parts as-is. They'd need to effectively redesign the cheesegrater at this point anyway, if they wanted to build more of (something that looks and works like) them using the suppliers+factories they have today.
The cheese grater design is one of my favourite designs for a tower ever and I've never owned a Mac.
The business partner has an old one that he uses because it has some old software he needs for design files.
When it eventually dies I'm converting it to a normal PC case and using it for my next build.
It's just a beautiful design.
What I really don't understand is why they didn't foresee this, and just updated their cheese grater Macs. After all, it's way easier to simply update a motherboard to fit a faster CPU than it is to rearchitect the Garbage Can design every time there's a new CPU and GPU architecture out there.
The only thing I can think of is that they've either just downprioritized a lot to focus on $new_secret_product, or that they've developed a new Mac Pro that for some reason didn't pan out, so they needed to start over again.