On the mandatory-or-optional front, publishing tools can’t realistically enforce the new field being semantically correct, they can only enforce syntactic validity.
This means making it mandatory is likely to make the field SNR worse rather than better, since rather than simply omitting the field when they haven’t fully thought through their licensing choices, publishers will be forced to pick a value that makes the tools happy.
By leaving the field as optional at a syntactic level, policy definitions can be left to the entity that has the most information on what matters for their use case: the folks actually downloading and installing packages (or the organisations they work for).
For the consumers most concerned about licensing details, even the new better specified field will only be viable as a first pass filter, since packages with acceptable nominal licences will need further analysis to check if their overall licence includes other terms (e.g. due to vendored libraries).
That doesn’t make the proposal useless, it just means making the new field syntactically mandatory would make it less effective at its intended purpose (allowing publishers that have put thought into their licensing choices to make those choices explicit in their published metadata) rather than being helpful.