Actually, it's not "the only new ride at the MK in years". Little Mermaid opened there recently. Yes it's a clone, but it's new to MK. Most WDW visitors don't also visit DLR, at least not on a regular basis. And Enchanted Tales with Belle is not a ride, but it is a new attraction with impressive AAs.
Unlike you, I don't know the future
. It must be nice to have such a reliable crystal ball. But if you look at the history of new attractions,
it is rare for a ride to open to a feeding frenzy for over a year, but then quickly diminish into unpopularity. I can't think of any examples. Can you?
Of course, it is likely to lose some of its momentum over time. That has happened to most headliners, including superstar attractions like Indiana Jones at
Disneyland, Space Mountain, Splash, etc. They enjoy sustained popularity, but they are not currently incredible draws like TSMM, Soarin', or, yes, Mine Train.
15 months is long enough that a lot of the people on the ride are not experiencing it for the first time. Yes, WDW gets a lot of first time visitors, but it also gets lots of people who visit one or more times every year. I've read countless posts from people who say that they visit that often.
I've also read many, many posts by parents who say that Mine Train is their kids' favorite ride. When the younguns love it, that's the kind of thing that will sustain popularity over the long haul. TSMM comes to mind.
Time will tell, but if you reread my post, I provided lots of examples of rides that could not sustain a lot of popularity despite being new -- and certainly not the crazy popularity of Mine Train.