First, what's being taught should be applicable to the real world. That means that if you are to produce useful and productive graduates, getting them familiar with ARM is much better than anything else out there.

Second, even if you had a similarly performing architecture, unless some major manufacturer with deep pockets picks it up and fabricates a chip around it, there's not much you could do with it outside of FPGA experimentation (for something that can mimic the power and peripherals of the Pi, with all its shortcomings, you're like to need a multi $k Virtex FPGA).

Third, ARM is friendly enough to its partners and the ecosystem that it won't be going away for a long, long time. It's a reasonably open company as far as documentation goes, and do their share of community contribution. ARM was smart enough to grant architectural licenses to the only companies rich and smart enough to make their own architectures (Marvell, Qualcomm, Apple, Samsung and a few others), so they can spawn their own variants while still retaining standard ARM compatibility.

I don't see any company seriously threatening ARM's dominance for at least a decade. If you want to do embedded systems or mobile (and soon servers) low-level, you need to know ARM.