They don’t just become useless. They become your slack capacity where their low wait times offsets the longer waits at marquee attractions so that you average out with the desired attractions per guest per hour.
That's misleading though. You can have a 60 minute wait for Mine Train, a 0 minute wait for Carousel of Progress and say the park wide average is 30 minutes and no additional capacity is needed. But that 60 minute wait doesn't feel any better to the guests and the 0 minute wait doesn't make Carousel of Progress all that much more desirable.
Take this as an extreme example: they could double, or even triple the capacity of the Magic Kingdom by keeping the place open longer. But advertising a 0 minute wait for Mine Train at 4AM, doesn't make the people in the 60 minute wait at 2PM feel any better.
This is part of the problem of not really understanding the complaints and what things like Fastpass and Genie were overall meant to do. The complaints get flattened to "the lines are too long" but in actuality the concerns are much more focused: it's that *this specific* line at *this specific* attraction is too long.
Think of it more like a top-ten list. Everyone comes to the MK with a top ten list in their head of attractions they want to see. Since there's a lot of overlap between guests, the lines for the top ten attractions will always be the longest. People will complain that the wait for those attractions is too long, and they won't care at all about the wait for the riverboat, the tiki room, the country bears, or any number of other high capacity attractions that aren't on the list.
And adding another attraction doesn't change this. The names on the top ten list may change around a bit, but ultimately people will always be frustrated that the highest demanded rides have the longest waits.
At any rate, I’d argue that a better solution than sinking millions into my magic (or whatever it was called) and then millions more into genie, is to build headliners with greater—much greater capacities.
Here's the bigger problem with this: there isn't really a solution to the high demand/high capacity attraction. It is a problem compounded by this basal instinct in humans to find greater enjoyment (and have higher demand) in physical thrill rides over shows/presentations. The longer/faster you make a roller coaster, the lower it's capacity will be. You can make another high capacity film/dark ride attraction, but the demand won't really be there.
The audience, more than anyone else, gets to dictate what type of attractions Disney builds, and they are advocating for, cutting edge technical design, exciting thrill elements, and almost no wait times. There's no easy solution to that.