Remember that you always see higher ridership numbers in years 2 and 3 after a ride has been installed, so we'll probably see even higher numbers in the coming two years.
Also, "only 50%" is a pretty loaded statement. Let's use simple numbers for this, just to keep it clean. If Maverick has 1,000,000 riders in one season, then Gatekeeper will have 1,500,000 riders. Sure, it's only 50% greater. But 50% is 500,000 people. That's a loooot of people. Even if we assume crew is hitting perfect intervals and getting to the max theoretical capacity (1,200 pph), to match that 500,000, Maverick would have to stay open for more than a month longer to hit those numbers. And that's an ideal case.