When is CompletableFuture the wrong tool now that virtual threads exist?

Async composition exists because blocking a platform thread is expensive. Virtual threads remove that premise: a blocked virtual thread unmounts from its carrier and costs almost nothing, so plain sequential code with plain try/catch, a readable stack trace and a debugger that works is once again affordable.

CompletableFuture still earns its place for genuine fan-out and fan-in, for bridging callback-based APIs, and for composing results that arrive from elsewhere. But "we used it so we would not block" should now be re-examined, and structured concurrency covers the fan-out case with better cancellation.

The nuanced answer to give: async is about the shape of the dependencies, not about the cost of a thread, and the cost of a thread was the only reason most codebases adopted it.