It is useful, but it also has weaknesses. For example, I’ve lost count of the number of times I’ve seen someone forget to close() an IO-based stream (eg File.lines). But probably for 99% of cases you could get away with slurping the entire file into memory and returning a list. The streams API is optimised for the 1% of cases that need the additional complexity, at the expense of worse ergonomics for the common case.

I get why it ended up that way. I think it would have been difficult at that time to get traction for adding FP features just as a convenience. So they needed to do all the parallel stuff to justify it. But, as I said, anyone I see doing serious parallelism in Java is not using the streams API.