Bank makes money by taking the deposit and loan to someone. and because for gold/premier account, they sort of have a deposit of at least HKD 1mil for loan to someone. that's how they can afford higher cost (with those better lounge and customer service) with the increased revenue.
But my personal opinion, never upgrade ur account unless you know the cost benefits outweight the opportunity cost. Think about it, with a HKD 1 mil of deposit, you can place a time deposit with another bank for better rate (and chances are these banks are not the same as your main operating bank). and that's the opportunity cost.