The Window Closes: Why I Switched to Kotlin Before the Codebase Got Too Big

I moved a project from Java to Kotlin in a single pass, back when the codebase was small enough that doing so cost almost nothing. The system I work on now is a different animal: hundreds of thousands of lines of Java, a monolith rather than a set of services. Converting it wholesale is the kind of gamble nobody signs off on. The option still exists technically, but the cost is now measured in quarters of engineering time rather than days. How much code existed when someone first asked about Kotlin decided everything that followed.

I have spent more than a decade building and scaling high-load distributed systems on the JVM, across fintech, security, and media infrastructure, and the pattern holds: you pay for a language migration at the start, well before anyone puts it on a roadmap.