10 new "bitcoins" could fork every day. Are we really going to expect coinbase to support them all? That's absurd.
I think it's pretty obvious that they're going to just issue the BCH to people eventually.
Here's another thought experiment: I am a now-extremely-wealthy bitcoin early adopter, and I am so happy at bitcoin's success that I am going to distribute $100 of USD cash per bitcoin to every person who can show me that they own a bitcoin.
To claim your $100, send me a Self-addressed-stamped-envelope proving your bitcoin ownership, and I will mail you back the cash.
Does coinbase have an obligation to send a bunch of SASEs to me? Are the coinbase customer's "owed" that $100?
I personally don't think so. I think that's the tradeoff you're making by having coinbase securely store your coins for you.