As you know, ONC can require any triggers it wants whenever/ifever they mandate support for the patient data feed. Historically when an IG defined multiple optional capabilities, ONC has designated their own set of mandatory ones to align with their priorities. Reconciliation with the published IG can happen (much) later or never. So I don't see lack of Argo-mandated triggers as a fundamental flaw. (Remember this spec came out of HTI-2 proposals that were themselves quite vague on use cases -- so instead of trying to pick winners or downselect data types, we described a general purpose approach that covers any uscdi patient oriented data. This should allow what you describe as "enrollment" to the extent that it is supported by uscdi -- and if it is not supported by uscdi then there is more fundamental work needed before onc can try to solve the problem.)
The fix is small: require the feed to fire on the resources that actually signal enrollment (Patient, Encounter, ServiceRequest, MedicationRequest, Appointment), and a whole swath of provider apps could finally run on FHIR alone. A glorious future awaits.