I'm mystified why you think it takes that long. There's a train every hour which takes 70-75 minutes to Gateway. Even allowing for waiting for the tram, it must take less than 2 hours. And you can book the tram as an add on to the train ticket.
The question is, how much of this £4.5m subsidy should be paid by the public purse and how much should be paid by the small number of passengers.