There is a small selection base boards prescribed by Google, that a manufacturer can built a Chromebook around. That base board covers pretty much everything AFAIK, except for touchpad, touchscren, pen input, etc.
I count around 40 base boards[1] that are still supported today, one year from now it's 37, two years from now it's 31. And you see much fewer variety in newer devices.
The x86 boards are very similar, you'll often have the same board just with a newer CPU/SoC. It just means you have one more device in the regression test lab, it won't necessarily create more work.
how is Google guaranteeing 10 years of updates on Linux while supporting newer and newer models every year.
In 10 years - Google's supported chromebook devices would exponentially blow up (versus Apple).
How are they pulling this off ?