Absolutely agree that this is a necessary feature - or some reliable scalable way of triggering an automation on creation/updates to activity records.
A heads up for anyone using the list-based approach: it only works in small-volume situations, once you start having high turnover in these processes they will start having multi-hour delays in being added to the lists; or failing to be added to the list before they are no longer eligible for enrolment and therefore being totally missed.