The bus was cancelled for lack of ridership
before the UPX will have even opened. I don't think the cannibalized ridership from a bus that ran every 40 minutes is going to pack full an entire train running every 15. The bus was useful because it went directly from the hotels to the airport, which is great for flight attendants on layovers.
I'm not saying that fewer people would be using it. Just not that there would be many, at least not enough to get to standing room only. The flight attendants on layover may take it, but baggage handlers or ground crew employees that have to travel there on a regular basis would probably find it prohibitive. I can't picture many sales for the monthly pass.