Then how about this: JimBob constructed a perfect copy of everyone's bank safety deposit box key and its contents, including the keyhole[1]. The contents of the copy are free for the taking if you can open it with the key.
Your bank happens to have an access control policy where they hold the safety deposit box key for you and you have to pick it up from them each time (after authenticating).
You ask the bank for your key so can go to the JimBob version and get your copied stuff. They refuse because of a security policy about taking keys offsite.
You offer to close the box account and take your (original) stuff out so that there will be no security compromise. They refuse because that would compromise the box and cost them a usable one.
You offer to pay for the cost of replacing the box with an uncompromised one. They still refuse.
That is the position that Coinbase is in. (Users haven't offered to pay for the replacement of the private keys that coinbase uses, but this is a trivial cost, and establishes the barrier that prevents coinbase from facing unbounded costs to deal with every fork.)
EDIT: And for an analogy that favors "Coinbase doesn't owe you anything": Assume we're under a gold standard. Assume that some goldsmiths just give you regular dollar bills as your warehouse receipt "since they're redeemable with the government banks anyway". Most people don't care much about the difference, but each goldsmith keeps a list of the serial numbers they gave out so they don't have to honor arbitrary demands.
Then (of course) the government goes off the gold standard. But some goldsmiths are like, "no, that's BS, we will still honor your claims, just bring us a dollar bill that's on our serial number list". So there's a mad dash to get dollar bills that were used as goldsmith receipts, and some people ask for their deposits back from their bank, but the bank (like Coinbase) insists that they only have to give you banknotes of their choice, not the original ones you deposited, which was all they ever promised.
(Modified scenario: you kept the dollar bills from the goldsmith in a safety deposit box, and the bank replaces them with new dollars and never redeems them for gold.)
[1] You can further assume that the copy is some debased, lower value version of arbitrary percentage and it doesn't affect the analogy.
Your bank happens to have an access control policy where they hold the safety deposit box key for you and you have to pick it up from them each time (after authenticating).
You ask the bank for your key so can go to the JimBob version and get your copied stuff. They refuse because of a security policy about taking keys offsite.
You offer to close the box account and take your (original) stuff out so that there will be no security compromise. They refuse because that would compromise the box and cost them a usable one.
You offer to pay for the cost of replacing the box with an uncompromised one. They still refuse.
That is the position that Coinbase is in. (Users haven't offered to pay for the replacement of the private keys that coinbase uses, but this is a trivial cost, and establishes the barrier that prevents coinbase from facing unbounded costs to deal with every fork.)
EDIT: And for an analogy that favors "Coinbase doesn't owe you anything": Assume we're under a gold standard. Assume that some goldsmiths just give you regular dollar bills as your warehouse receipt "since they're redeemable with the government banks anyway". Most people don't care much about the difference, but each goldsmith keeps a list of the serial numbers they gave out so they don't have to honor arbitrary demands.
Then (of course) the government goes off the gold standard. But some goldsmiths are like, "no, that's BS, we will still honor your claims, just bring us a dollar bill that's on our serial number list". So there's a mad dash to get dollar bills that were used as goldsmith receipts, and some people ask for their deposits back from their bank, but the bank (like Coinbase) insists that they only have to give you banknotes of their choice, not the original ones you deposited, which was all they ever promised.
(Modified scenario: you kept the dollar bills from the goldsmith in a safety deposit box, and the bank replaces them with new dollars and never redeems them for gold.)
[1] You can further assume that the copy is some debased, lower value version of arbitrary percentage and it doesn't affect the analogy.