If you're in your fifties, a single decade is about all you need to take you through to retirement age.

To be honest, I would've said going deep on JavaScript right now (React, React Native, etc) would give you a shelf life of at least five years.

Kotlin seems like a bit of a random recommendation - it's not in the TIOBE top 50, and a quick search for Kotlin jobs on indeed.com (the first job search site I found from Google) reveals several orders of magnitude fewer jobs than for anything mainstream (hell, even Haskell beats it comprehensively). Perhaps it's the next big thing though!