That used to be the case, actually life cycles in earlier years were sometimes even shorter than that (apparently it was only a year and a half back in 2007 -- http://reconanalytics.com/2015/02/2014-us-mobile-phone-sales...).

But several articles seem to point out that the upgrade cycle is slowing down -- up to 2 and 3/4 years according to this source (https://www.npd.com/wps/portal/npd/us/news/press-releases/20...) for instance.

I think the second link is correct that 5G might offer some incentive to upgrade, but else wise there's just less reason to constantly upgrade your phones these days. The delta between an iPhone 7 and an iPhone 8 is not that huge versus the delta between an iPhone 3GS and an iPhone 4.