4/4S and 5/5S were not on the same chassis. 5/5S/SE were.Apple hasn't released a new Industrial Design in an "s" year iPhone, so you should plan on seeing it next year. The 3g and 3gs only had two years, but the 4/4s/5/5s, the 6/6s/7/8 and now X/Xs/11 Pro/???? each had 4 iterations on the same basic chassis. Expect a new ID in the 2021 release.
The SE came out the same year as the 6, and the 8 came out the same year as the X. Even though they had extra iterations, we can still expect a redesigned flagship model next year even if they offer a lower tier model on the same basic X/XS/11 chassis.