I doubt that 5 years from now the norm for *desktops* will be 12 disks on a RAID and 8 threads writing on it.
I think <8 threads writing concurrently is going to be the normal case for desktop systems for quite a while, and I'm not sure any amount of hand-waving is going to dismiss XFS being 50% slower compared to ext4 in your benchmarks in the case that is important to desktop users.
No, 3 seconds compared to 2 seconds doesn't sound so bad, but I'd expect that to mean also that the operations that now take 2 minutes on my ext4 to take 3 minutes on XFS. A 50% difference certainly means it is appreciably slower.
Simply put, I don't think scalability in the way it is exercised in your tests is relevant to desktop users currently or in the near future. More than 8 threads may be imaginable in the future, but 12 disks is not. At least currently I care way more about a single-threaded workload being much slower on XFS (per your benchmarks). It *is* the absolute numbers that matter, after all.