And like with page sizes, the big problems with cache line sizes is not when people design things against a specific line size, but the (much more numerous) cases where they design something that only works well for a fixed size but they don't even know that, because they literally never thought about it, and it worked fine because every machine they used was similar.
It doesn't matter what apis you were provided or what analysis you did, you don't know your software works with different hardware until you test it on it.
But when someone releases a new CPU with a larger or smaller cache, all old software continues to work.
Secret page size would offer the same benefit.