"Major upgrades and changes" for the "full incremental" model? What are you talking about?
You'd need to define "major upgrades and changes"
If one defines chipsets upgrades as major, then no. Those actually happen in the "S" models. The 3GS, 4S, 5S (and hopefully 6S) all brought significant internal upgrades to the previous models. The numbered upgrades were more minor internally, major externally. The exception would be the iPhone 5 which did double CPU performance over the 4S, but GPU was not as big of a jump and the camera was almost identical to the 4S minus a few tweaks & sapphire cover.
If one is talking about redesign primarily, then yes, you're right, very clearly the numbered generations bring the major physical alterations (case redesign among other things). If one is referring to "S" models getting "minor" upgrades, with reference to case design, then yes, you're right! That is quite evident looks-wise.
Apple has to justify "S" models and bring incentive in other ways, given that it looks mostly the same. So they tend to have the new "big" features (Siri, Touch ID, 64 bit architecture, Force touch potentially - a completely new interface isn't "minor") that get incorporated into the next numbered model along with a minor spec upgrade (5 is the exception to this, but every other numbered model from 3G, to 4 to 6 were all minor spec bumps over previous generation. The 3G was essentially the exact same as the Original iPhone; the 4 and 6 brought very small CPU/GPU bumps over the 3GS and 5S respectively....in the case of the 6, it was arguably more disappointing because the RAM amount stayed the same)
This whole "S models are minimal, minor" upgrades is getting tiresome to read. Its just ignorant given how big some of the features introduced in the "S" generations have been.
Every generation bring its own set of upgrades, be it external or internal. They are legitimate upgrades in their own right. Even though the 4 was not a big leap over the 3GS internally, RAM aside, the front facing camera and retina were huge upgrades. Even though the 5S did not look much different from the 5 outside, Touch ID and 64 bit architecture - along with a co-motion processor - were all significant upgrades.
And those who are going from contract to contract, often 2 years, tend to receive a worthy upgrade to their older iPhone. 3G -> 4 -> 5 -> 6 are all solid jumps, and worthy leaps. As are 3GS -> 4S -> 5S -> likely 6S will be *(hopefully 6S continues this trend...)*
Those who upgrade year-to-year, well its going to be more a of a minor upgrade regardless if its a numbered or S model generation.
Please lets let those who look forward to S models, or upgrading from 3 year contracts, or coming from an even older iPhone, please can we just let them look forward to the latest iPhone? Why people feel the need to poo-poo that by incorrectly saying how "minor" S models are as an upgrade, I'll never know.
Who's poo-pooing? Feel free to be excited about any model refresh of any phone. Please don't let opinions of others like mine stop you.