I'm a pretty heavy TS user and I was pondering the same thing. I've not been able to find a motive for allowing numbers as interface keys, but Symbols actually do make sense since, turns out (I'm always learning something new), you can use them as object keys in ES6: https://www.keithcirkel.co.uk/metaprogramming-in-es6-symbols...

That post outlines a few use cases which are admittedly outside of everyday JS/TS usage but I can see them as quite powerful when, for example, building frameworks and libraries that expose an API. I'd offer real-world examples of my own but I'm still working it out for myself .)