No hash power cut-off. Imagine we implant a secondary ASIC-resistant cpu/gpu friendly algo (like ProgPow) and define two parallel difficulty adjustment systems which regulate blocks mined by sha2 at 11 blocks per every 2 hours ratio while gpus are set to generate 1 block in the same window also suppose we use a gradual re-adjustment policy by introducing a decrease/increase strategy for above parameters. See? no cut-off!
How many hard forks will that take?
What would make you so sure that Bitmain will not make new ASICs for the new mining algorithm?
It is just ONE hard fork. The ratio is adjusted deterministically by virtue of the protocol, just like halving in legacy bitcoin.
Good luck in finding a solution.
Solution is
almost ready, implementation is the phase.