Value 60 is not an arbitrary number. It makes the kernel reclaim 140 file pages for each 60 swapped-out anon pages. This proportion is supposed to reflect the relative performance cost between sequential file access and random swap I/O on HDD. For SSDs, the value 100 is more appropriate.

> I don't see a reason why the default shouldn't be 0 then on modern systems.

The zero value makes reclaim very unbalanced and is not appropriate for generic systems. This will cause the kernel to discard all the page cache, slowing down the system with uncached I/O, while keeping all unused anonymous pages in memory.