Another voice on whether or not APFS will be mandatory in High Sierra comes from this MacRumors report on the High Sierra Developers Beta.

Originally Posted By: MacRumors
When installing High Sierra, it will convert to a new, more modern file system called Apple File System or APFS. APFS is safe, secure, and optimized for modern storage systems like solid-state drives. Features like native encryption, crash protection, and safe document saves are built in, plus it is ultra responsive and will bring performance enhancements to Mac.

That appears to imply the file conversion in High Sierra will be as it was in iOS 10.3 — unannounced, automatic, and unobtrusive. The public beta is supposed to be released this month and when it does I will let you know…
  1. if the conversion occurs
  2. if the installer notifies of the change
  3. if there is an alternative offered to retain MacOS Extended (HFS+)
  4. if the conversion appears to take extra time during the install
  5. are all attached drives converted or just the boot drive
In iOS 10.3 the answers were...
  1. Yes
  2. No
  3. No or N/A
  4. No
  5. N/A

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein