A commodity must be fungible -- i.e. that would imply BTC and BCH/LTC were economically interchangeable.
They aren't. Bitcoin's blockchain has a longer proof-of-work than BCH. Honest nodes will only mine the longest blockchain (per Satoshi). Therefore any blockchain shorter than Bitcoin's will ultimately collapse in the long term, making them unsuitable to be considered commodities.
Exactly, so BTC and BCH would be considered different commodities. But BCH can never win provably in the long-term, so therefore would not be able to qualify as a functioning commodity.
They aren't. Bitcoin's blockchain has a longer proof-of-work than BCH. Honest nodes will only mine the longest blockchain (per Satoshi). Therefore any blockchain shorter than Bitcoin's will ultimately collapse in the long term, making them unsuitable to be considered commodities.