What makes you think that a computer with a MIPS or Sparc cpu, in stead of an Intel cpu, does not adhere to the Von Neumann concept ?
The letters PC stand for: Personal Computer. Personal Computer is short for "IBM-compatible Personal Computer". And "IBM-compatible Personal Computer" means a small/affordable computer with an Intel CPU of the x86 cpu-family, running an Operating System made by Microsoft. Just because of that definition, it is impossible to make a PC that does not have an Intel cpu inside.
Now if you wanna know, have there ever been small/afforable computers with a CPU that is not an Intel cpu, that is a completely different question ? Sure. Early on (in the eighties and nineties) there were a lot of small/afforable machines you could buy from different vendors. But they were called "workstations". And they were a lot more expensive than PC-hardware. During the first decade of my career, I've always had a SparcStation on my desk. At first with a Motorola 68000-family cpu, later with Sparc cpus. But around 2000, when I moved from a huge company to a small startup, I switched to using Linux on PC-hardware. That's very very similar to using Solaris on a SparcStation. But at a fraction of the cost. With current low prices for PC-hardware, there's very little reason to have something on your desk that is not basically PC-hardware. The market is just too big. And compatibility and standards are so awesome that you can make any combination of hardware and software work.
Last remark. About the Von Neumann architecture. Yes, people can make computers that do not sure the Von Neuman architecture. But that would make their use a lot more awkward or complex. If someone would make devices, or chips, that do not use Von Neumann, you wouldn't call them PCs. You would not call those chips CPUs. You would just call them ICs. Or dedicated-function chips.