And in the meantime, they did use those first two years of Skylake for the 12" MacBook; the next update to the MacBook Air was after the last update the 12" MacBook ever got. For a while, the 12" MacBook was the more premium, thinner and lighter alternative to the MacBook Air with more advanced technology (and could plausibly have been construed as the intended successor to the MacBook Air), then in 2018 they merged back together with the introduction of the first MacBook Air with a Retina Display.
They sold old hardware for the same price 3 years later as if it was a premium product. They didn't really have an excuse, they've been the most valuable public company on earth since like 2010.
The three-year gap in processor updates you're complaining about disappears when you recognize the 12" MacBook as an attempt to move the product line in a different direction, which Apple partially backtracked on after a few years. That course correction was quite a bit quicker than for the Touch Bar MBPs and the trash can Mac Pro.
and if my grandma had wheels she'd be a bicycle. As far as I can glean this was never something that they intended to do.