Another advantage of building your own VM vs an ISA is you can tailor key aspects for ultimate efficiency.

RISC-V has no built in heap facility and leaves this up to the OS/software level. For normal computing, this makes total sense, who cares how you want to manage the heap. But in a metered environment like a blockchain, this adds a lot of overhead as each operation must be gas metered for every transaction.

The FuelVM, designed for blockchain and metering, has heap facility out of the box for this exact reason, avoiding having to pay for this every transaction.

Sure, in RV you can maybe add this facility in the ecal but then you might as well just use the FuelVM.