In fact, `tail -f | grep` works the way they expect. What doesn’t work is `tail -f | grep | grep`.

It’s hard to argue that grep isn’t supposed to work like this when grep tries to work like this. It’s not a fundamental tension, it’s just that isatty(stdout) doesn’t always tell you when you’re running in an interactive terminal.

That's a good point. If the proposal is to propagate that all the way through the pipe chain I'd support that. My gut negative reaction is to the complexity of adding flush timers.