Apart from being way out on your numbers (a million or more?) the 720 only applies to those Spanish citizens or residents.
The penalties for not completing one are extremely high. Even though the EU doesn't like the 720, they haven't banned it but have ordered Spain to reduce the fines which were, at the outset, sometimes coming to more than the amount they had in overseas accounts.
If you have completed the form once, there is no need to complete one every year unless the amount in any band increases by €20,000 or more. Simply state no change.
Anyone not completing one would be rather foolish as the fines are still at a crippling level.