But you have certainly been using variadic functions or classes, e.g. everytime you call std::make_unique(). Someone needs to write those :)

That's a fair point. It wasn't a criticism of the feature - more confusion about what exactly it's supposed to be good at solving. I'm never clear if I should be reaching for it. make_unique() is a good example.

Just as a simple example, Optional and containers can use variadics to forward arguments to emplace-construct their contained objects.

If you do not need variadics that's fine, you do not have to force them where they don't belong.

You can't do type-checked multi-dimensional containers like tensors without variadics. You could only manually create Tensor1D, Tensor2D, Tensor3D... types. You also couldn't implement tuple types without them.

Variadics are rarely useful, but when you need them (generally in highly generic library code) you really need them. (I don't know C++, but am familiar with the pain of not having variadics in Rust, and the piles of macros required to work around it)