Todos debemos morir algún día.

surely just easier to say… Todas moriremos un día or todas vamos a morir un dia. this structure just sounds really unnatural.

Certain situations require nuance or emphasis. In my native level American English I roughly translated the sentence as, we all must die one day. Saying, we all will die one day or we are all going to die one day, are just options to express the same idea. The translation given isn’t even one of those options. Hahaha… The flexibility of a language to express our ideas is a beautiful thing.