Yeah, if they play it right they can be one-size-fits-all solution. A question is if they would take risks with such approach as their motto is to "implement stuff that is winning", i.e. copying things others did and that worked well (in commercial sense) and avoid experimental stuff that doesn't bring money in.
Kotlin right now is a serious competitor for a full stack, any device language. An alternative may be F# which has Fable for Browser, Xamarin (mobile) and .net core for server. But Kotlin looks more integrated to me.
Coming from a C#/F# background, Kotlin is really a joy to use.