Difference between 6s and 6 were also small and so are the differences between a Samsung Galaxy S7 and a S6, for example. I'd venture that is very rare these days to have a phone that justifies buying the immediate next generation one. Phones are mature enough. Good ones are really well built and will last a couple generations just fine.

As long as this is an industry-wide problem, Apple has nothing to fear. It would be a problem IF people who have waited three years to change their iPhone decide to go for a different brand.