Apple very strongly advises that actions available via 3D Touch must also be available via some other mechanism. 3D Touch right now is used for shortcuts (e.g. on the home screen), or previewing things (e.g. Peek) where the Commit behavior is the same as what you get when you actually tap on the item instead of 3D Touching it.

This is important because many users, especially the elderly, do not have the physical ability to press hard on the screen.

It doesn't really require that much force. Subjectively, it feels to me like Peek requires less force than pushing the Home button, and Pop requires a bit more, but not significantly more.

Also, under Accessibility, there is a way to control the 3D Touch sensitivity, so it requires less (or more) force to activate.

That said, for people who use alternative input mechanisms (whether it be various forms of assistive touch, or just using VoiceOver), AFAIK they can't 3D Touch (well, you can 3D Touch with VoiceOver, but only if you can see where to push; there's no way to trigger 3D touch on the highlighted element if it's not under your finger).

Not much in the short to medium terms. Apple are still selling lots of iOS devices that don't have 3D touch hardware support, and will probably continue to do so for several years to come, so even iOS only apps will still need to offer alternative UI options for the lifetimes of those devices.

Liken it to a right-click on a two-button mouse interface. It just ads shortcuts (as stated below). You might cut/copy/paste with right-click in a spreadsheet, but it's also available on the File menu, keyboard shortcuts, etc. Need it? No. But I can see it being incredibly convenient and time-saving in many contexts.