I think you are correct. An accountholder cannot be forced to complete these forms. The bank may close the account if one doesn't, but they are not required to do so by either the CRS or FATCA protocols. You will notice that the advice from the bank states that they may close your account, not that they will close your account. But at the end of the day, it is up to the bank to decide policy on this.
I think that for the accountholders who do not provide the information and sign the forms, the banks are supposed to aggregate the balances and transaction information from these accounts and report the aggregates without any detail regarding the accountholders. I think these accounts are labeled "recalcitrant accounts". The banks are not penalized if they are unable to obtain all information from all accountholders.
This is my understanding, but I did not work in the banking industry. Perhaps someone with closer knowledge can further clarify.