I agree that more passengers would be needed before a people mover is warranted. That need for more passengers is also a reason I don’t think the money for a commuter rail should be spent to make that connection.
If you let the commuter rail do it’s own thing and make it as efficient and cost effective as possible, then it becomes the airport authority’s responsibility to take advantage of it. Let them do their expansions and get their numbers up. The I-40 ramps are being improved now which should keep traffic acceptable for the foreseeable future. If the rail is a success and traffic becomes a bigger problem in the future, then RDU could request funds for a people mover on their own. Let them get money from all the surrounding counties and put in a people mover that they would control. That way they can decide how it works with their parking and bus situation, they can set the schedule, select luggage specific pods and charge whatever works for the ride.