My two cents: Having waited in an only-25-minute-line for Millennium Falcon at DL a couple of weeks ago, I can testify that the attraction moves people through surprisingly quickly (the line was almost to the entrance, but it moved constantly, and the queue was so well-done and full of details that it was a pleasure to move through), which reflects both a respectable capacity, AND the lack of Fastpass. At Slinky, the wait is going to be longer because its capacity is tiny, and the vast majority of people riding it have FP, so those in standby can't avoid a long wait even when the standby line is short. It's not that Slinky is any better or more popular. In fact, having experienced MFSR without Fastpass bogging down the line, my personal wish is that it never starts distributing Fastpasses.
While I think Disney shot itself in the foot by opening Galaxy's Edge without its main attraction, and by cutting out a table-service restaurant and other elements that would have made it more of a draw to guests of all ages, I don't necessarily think the lack of crowds is reflective of a lack of interest or an indication that MFSR isn't a good attraction. I suspect that many guests are simply waiting to visit until the land is "complete," and don't like the idea of walking all the way out to a far-flung section of the park when the only thing you can do there (without spending additional money) is just one attraction.
