I don't think there's really a credible alternate reality where Moore's law just stops like that when it was in full swing.

The ones that "could have happened" IMO are the transistor never being invented, or even mechanical computers becoming much more popular much earlier (there's a book about this alternate reality, The Difference Engine).

I don't think transistors being invented was that certain to happen, we could've got better vacuum tubes, or maybe something else.

As someone has brought up, Transputers (an early parallel architecture) was a thing in the 1980s because people thought CPU speed was reaching a plateau. They were kind of right (which is why modern CPUs are multicore) but were a decade or so too early so transputers failed in the market.

When MC68030 (1986) was introduced, I remember reading how computers probably won't get much faster, because PCB signal integrity would not allow further improvements.

People that time were not actually sure how long the improvements would go on.