The example I think is compelling is JDBC.

Nearly all SQL Database access goes through it and it won't be rewritten in any practical time scale. It can't be meaningfully async without a rewrite and kotlin's coroutines can't change that.

With virtual threads that synchronous api can be used at the performance level of what a hypothetical JDBC rewite would give.

(Not that you want to hit your DB that much, but you know what I mean. You don't need to specially wrap or bundle apis to get into an async call juggle.)