I think the online limit just prevents it from being oversold. The last thing you want is everyone buying up the 4:30 slot and being in the position of either cycling for 4 hours past closing (not going to happen) or having to process a lot of refunds. I have seen someone who purchased online right before closing get turned away by the ops team (but that was when they had fireworks on, and the ride had to be shut down prior to them starting)
You've still got wristband sales in the fairy shop, and ops team can take card payment at the front of the queue too. It quite frankly couldn't be easier to purchase if they had a paywave terminal mounted to the seat. /s
I think at $25pp its a great one off. I won't ride it every time but will fork out when i'm bringing someone to the park that has never done it. If it were discounted off peak. BOGO or similarly priced to sell more than one for a discounted total, i'd be all for it. I think $10 or $15 per person, or in a multi-buy (say, buy 4 for $100, get two free) would ultimately see more revenue and less empty seats.