Yep, the IBM PC clones are what opened things up for x86 - you could get PC clones for a lot cheaper than Macs or even IBM branded PCs. I suspect that if IBM had decided to not allow clones (either by design or by lawsuits) that the 68K family would have been viable for longer (and/or we would have seen Macs transition to the 88K). There would have essentially have been main 2 choices for a while:IBM PC or Macs. Which would also have meant that Intel wouldn't have completely dominated the processor market - it would have been more of a 2 player CPU market.
We also had the Amiga and the Atari ST machines on 68K then as well, so without PC clones they would likely have become the low-price choice for a while.
I mentioned this in another comment, but the presence of interchangeable clones created a form of distributed long term support for business software. This was the killer feature that propelled business investment. Companies invest a lot in tailoring off the shelf software and custom software. Knowing that you have the ability to switch to another vendor made the decision a no-brainer for most businesses.
We also had the Amiga and the Atari ST machines on 68K then as well, so without PC clones they would likely have become the low-price choice for a while.