I don't have a single source about this. But some hints are: 10 years ago every µC manufacturer had it's own core and instruction set and basically sold them ex

I don't have a single source about this. But some hints are: 10 years ago every µC manufacturer had it's own core and instruction set and basically sold them exclusively (maybe apart from some 8051 stuff). They were AVRs, PICs, V850, PPC, etc. Now when you look that the portfolio of those manufacturers they nearly all have at least one product with an ARM core in their portfolio. Some might even already stopped releasing new chips with their proprietary cores. Others have done that not yet but are planning to do this in the future (I've heard of at least one). Another hint is when you actually get in touch with a lot of complete product designs. The amount of ARM cores that you will see in there seems steadily to incline.