I'm not sure I buy that, you can have a lot of competition on the same ISA. After all on the desktop/server x86 has been king for a while, and on embedded architectures ARM has all but taken over the competition. Yet there's significant competition on both fronts.
Meanwhile attempts at creating new ISAs over the past couple of decades has been met with relatively little success, or only in very specific niches. Itanium being an obvious example. People talk about RISC V a lot but in practice it's not really making a dent into the ARM market share yet.
These days competition appears to come mostly from companies reimplementing and extending existing ISAs with better performance.
I would just like to add in that RISC-V is still a lot of talk because there are still some important standards that really need closer attention before we start mass-producing server CPUs. Specifically, the Platform-Level Interrupt Controller[1] and the Vector Instructions[2]. The former is super-important because we now know interrupts/traps can be used in Spectre/Meltdown style attacks, especially if your chip has something like a DSP/iGPU, which can be used in covert side channel attacks. The latter will be important for the same reason that SSE/AVX is important today.
Just my opinion, but SiFive has been a huge driving force in even giving RISC-V a shot at breaking into server space. Most other companies (Google, Nvidia, Western Digital) have invested in RISC-V with the apparent intent of focusing on building microcontrollers/embedded processors. AFAIK, there's only one player who is investing in building RISC-V server-class CPUs and that's SiFive.
Please correct me if I'm mistaken! I've been following RISC-V pretty closely for the last year or so and there's quite a lot to catch up on.
Meanwhile attempts at creating new ISAs over the past couple of decades has been met with relatively little success, or only in very specific niches. Itanium being an obvious example. People talk about RISC V a lot but in practice it's not really making a dent into the ARM market share yet.
These days competition appears to come mostly from companies reimplementing and extending existing ISAs with better performance.