I don’t think ambiguity is past its due date. It affords the US a lot of strategic flexibility, which is a good thing. Even if you strongly support TW, as I do, it is easy to see the value in a position which affords the US more wriggle room. Beijing doubtless plans for US intervention in any Taiwan contingency anyways, so I am unconvinced that abandoning the policy of ambiguity would meaningfully impact how Beijing views its options on Taiwan.