A lot of the interest behind open ISAs like RISC-V isn't from a cost perspective, because most people recognize that the cost differences are negligible.
They want to avoid the Intel scenario, where a company controls an ISA that industries depend on, and can force their ideology on implementing silicon for whatever ends they want.
ARM, being proprietary and controlled by the company can do the same. If you want a specific part built, you need the permission of ARM to do anything involving the ISA.
If you are going to build all civilization on the top of an ISA, it can't be reasonable to subjugate that ecosystem to the whims of one board of directors.
They want to avoid the Intel scenario, where a company controls an ISA that industries depend on, and can force their ideology on implementing silicon for whatever ends they want.
ARM, being proprietary and controlled by the company can do the same. If you want a specific part built, you need the permission of ARM to do anything involving the ISA.
If you are going to build all civilization on the top of an ISA, it can't be reasonable to subjugate that ecosystem to the whims of one board of directors.