The BSD approach at the time was to require setuid access for programs like ps to be able to read the kernel memory space via /dev/kmem to produce a running process list.
The stupidity of that is orthogonal to whether to use procfs or another sane, but less plan9 approved design like a syscall interface or an ioctl.
I'm reminded of another thing that used to be file based but moved away, and towards syscalls: random number generation. One criticism of a /dev/random approach I've seen is that open(2) could have some fringe error case (descriptor table too big?), and you don't want your secure RNG to bail on you. In particular lazy initialization of a secure RNG where the caller may not be able to check for errors.
I'm reminded of another thing that used to be file based but moved away, and towards syscalls: random number generation. One criticism of a /dev/random approach I've seen is that open(2) could have some fringe error case (descriptor table too big?), and you don't want your secure RNG to bail on you. In particular lazy initialization of a secure RNG where the caller may not be able to check for errors.