More likely if the FDA was properly funded these things could get reviewed more often and this wouldn't be an issue. Not updating allowed ingredients in over 20 years doesn't point towards a lack of flexibility, its debilitation.

This isn't really the issue, most of the cost of reviewing new drug applications is covered by user fees. And most of the cost and time required for getting a drug approved is in the clinical trials. FDA resources aren't really the bottleneck, the FDA is generally faster than its counterparts in other countries.