It's really a situation of the 90/10 rule, you'll get the first 90% of the benefits from 10% of the work and then it's many many incremental changes that gets progressively more expensive.
Also in the case of Erlang the language model will reward these first steps even more than most subsequent optimizations because much Erlang code in general don't have much tight loops of the kind that are prominent in benchmarks were a good register allocator would provide huge wins.
More on the 90/10 rule we need to remember that these expensive JIT's are very complicated with optimization levels and interpreters combined with tons of GC options whereas here they explicitly just dumped JIT-interpreter cross-calling to simplify the design as well as a more straightforward internal memory model with less complicated edge cases.
Also in the case of Erlang the language model will reward these first steps even more than most subsequent optimizations because much Erlang code in general don't have much tight loops of the kind that are prominent in benchmarks were a good register allocator would provide huge wins.
More on the 90/10 rule we need to remember that these expensive JIT's are very complicated with optimization levels and interpreters combined with tons of GC options whereas here they explicitly just dumped JIT-interpreter cross-calling to simplify the design as well as a more straightforward internal memory model with less complicated edge cases.