As I see it, the ARM instruction sets (note the use of plural) are a moving target (look at Cortex chips with Thumb2 only). They are far from RISC if you count them, and the instruction encoding is horrendulous.
It's like saying "why not target Windows? I doubt that's going anywhere soon"
With FPGAs, your only risk is that the manufacturer discontinues the chip you targeted, and you can then target another brand with the same (or nearly the same) HDL code.