There are two things the demo will tell me.
1) Are the graphics in FIFA 17 that much better than they were in FIFA 16.
2) Will my PC be able to handle the new game.
I tend to skip years with my sports gaming and right now my gut tells me to wait a few months (or possibly the entire calendar year) until FIFA 17 goes on sale. That's what I did with FIFA 16. I only recently picked the game up on sale. But you never know. The graphics in FIFA 17 could be so good to make it a must buy. Only time will tell.
PS: I have 2 PCs at home. The 2 year old PC located in my office that I use for internet browsing and casual gaming. It has a decent 960 GPU and it's this PC that I presently run FIFA 16 on. Then I have a 2 month old new purpose built VR sim racing rig I use in iRacing and Project Cars sporting the 1080 GPU. The purpose built sim racing is a monster and it will easily handle anything FIFA 17 throws my way. Problem is, do I really want to be playing FIFA while I am sitting in a sim racing cockpit? I will if I have to. But I'd prefer to play FIFA with the older computer in my office. Heck I might even consider slapping something like a 980 GPU in the older PC to get a little more shelf life out of it.