That is subtly incorrect - what they are is "cheap generic x86" (or x64 now), that's why all those SGIs and Suns were dumped, the price/performance of the hardware. Linux just happened to be the x86 Unix around at the time. It could just have easily been FreeBSD or (if Sun weren't so short-sighted) Solaris x86.
Would anyone have ditched IRIX to run Linux on their MIPS kit? Of course not because it was never about the OS in this space, it was about FLOPS/$.
They were afraid that x86 hardware would eclipse SPARC hardware, so development of Solaris x86 lagged behind "real" Solaris, and the HCL was always very small. They wanted to sell you expensive hardware, not a software license.
What actually happened was that x86 hardware eclipsed SPARC hardware... And organisations that would have been happy to refresh the hardware and keep the same OS and just recompile their apps found that they couldn't, so they jumped to Linux instead... And the rest, along with Sun itself, is history. I was working for a big Sun customer during this transition, we begged them to make Solaris x86 a first-class citizen, and we had a lot of support within Sun too but the SPARC Mafia won the day and with regret, our next big order was (IIRC) with Dell and SUSE...
Would anyone have ditched IRIX to run Linux on their MIPS kit? Of course not because it was never about the OS in this space, it was about FLOPS/$.