Here's my opinion. First, the 16's ain't going to disappear anytime soon. Still lots of 15's available. What makes people think it is going away soon? And even if it was (which I still doubt), deal with it then. I'm not spending money now on something I don't need or only may need maybe 10 years from now.
Second, 16 inch tires are less expensive than similar sized 17 inch tires.
Third, when you wheel, you want as much tire as possible. So a 16 inch 35 is better than a 17 inch 35 which is better than a 18 inch 35...More tire, you can air down and the tire will flex better.
And last, you already have 16" wheels. Why would I buy 17 inch wheels just to put on 35's that I can already put on my 16's?
JMHO