OS X is as old as the classic Mac OS was when it came out

I'm not sure why the article is speculating about a "seismic" change. Apple (and Microsoft and Google have followed) has established a pattern for their OS evolution: yearly incremental updates.

They had established that same pattern before too, until they released MacOS X and created a seismic change.

What the author is saying is that we should be coming up on a new chip for the Mac pretty soon, and if history is any guide, we may also be upon a brand new OS.

That being said, I don't think we'll see seismic shift, unless you count the possible unification of iOS and MacOS a seismic shift.

From the article: "I doubt we will see a seismic transition to a new Mac OS"

This doesn't feel like the same speculation that you read?

I would honestly love to see a ground-up rebuild, but not the kind that would probably happen. OSX is getting creaky; it seems like it's had an increasing number of integral bugs lately, and the "redesign" that came with Yosemite felt more like a rushed coat of paint than a substantial rethink of the UX.

Here's what I'd like to see: a fresh desktop OS that actually innovates in the space. Desktop UX innovation has slowed drastically in the smartphone age, and for Apple it's come to a screeching halt (Windows - dare I say - is now the leader in desktop innovation). The fact that OSX is the only modern desktop OS that doesn't have window snapping out of the box is simply shameful. Even Ubuntu has it now, without any Gnome extensions.

Despite all this, OSX still has a huge foothold as the developer platform of choice - especially for web - because of its combination of quality hardware, Unix-ness, and relative stability (though all of those are becoming harder and harder points to defend). Apple should take advantage of this and throw power users a bone. We've been waiting for years on end. Stop working on useless things like the Touch Bar and start rethinking what a desktop operating system can be in 2018.

Unfortunately, what would probably happen if they built a new desktop OS is iOS with mouse and keyboard support, and developers would leave in droves.

Regarding: "I doubt we will see a seismic transition to a new Mac OS"

First of all, you can observe a drop-off in quality (-control) with every new upgrade. OSes should be stable and not layered with new destabilising os-features every year. Since Tim Cook took over, the apple products were mere Box'es and not new new internets. I don't think they will come up with something "seismic" big.