If it's a huge concern, keep waiting till it's not, if ever. You won't get to enjoy the features but...
You can always buy on the phone company or Apple 24 month plan. These let you upgrade every year.
I buy on the same plan but replace after 2 years and a few months. Gives me a little time to enjoy a lower phone bill. Then I decide I need a feature and go check it out ... a new iPhone follows me home and... we all know the rest.
The iPhone is now so good I don't feel the need for the latest and greatest. The only reason I bought an Xs is that I wanted the dual-lens camera in a smaller body (and I wanted Apple to work out any issues in the X). Otherwise, I would have kept my 7 Plus—which I did anyway and now use it as a stereo recorder (think of it as an oversized iPod Touch 6 with truly decent battery life).