You don't have to be a billionaire, I got a Curta mark I for ~£850 on ebay and it's like the world's greatest fidget toy.
Whatever patents that they had have to be expired, I kind of wish someone would make reproductions. I know there's the 3D printed ones, which are cool in their own right, but since 3D printers aren't super precise the parts have to be huge to compensate. I want as close to a one-to-one reproduction as possible, but I guess there's not much money in it.
The problem is that if I bought one now, it would simply be a toy and nothing else. That's just a bit more than I'm willing to spend.
I could probably get one of those cash registers to play with for not a ton of money, but my house isn't huge and it's hard to justify the space.
Add in the fact that authenticity is part of the appeal, plus the fairly expensive process to make a decent replica, it’s not shocking that no replicas have emerged, even though cheap-ish CNCs mean it’s probably easier to do than it ever has been.