5% is way too little to risk mainlining over
They have engineers dedicated to 0.5-2.0% improvements in the kernel
All in all, while a good Phoronix benchmark is what can make it supplant all other Linux filesystems, I appreciate the security concerns raised by maintainers, and I agree that a better approach would have been to use the default code at first, and seek advice on how to improve its performance. Thankfully, it looks like that is where it is going now.