Just because any race is not sold out does not mean that runDisney is looking to get rid of it altogether. If a race can hold 18,000 runners and sells 16,000 bibs is runDisney worried? After all, at least some of those 16,000 are going to buy park tickets, stay on property, buy food on property, buy race merchandise, etc. Especially because there is no guarantee that the new theme will perform any better. They noted this year that 13,000 runners were in the Star Wars 10K and 16,000 in the Half. Based on course crowding and similar numbers in other years, those numbers are close to capacity.
I'm not convinced that the Kessel Run is going away either. In The Force Awakens, Rey refers to the Falcon as the ship that made the Kessel Run in less than 14 parsecs before a grumpy Han corrects her to note it was less than 12 parsecs.
It's far more likely that the slight theme change from rebel to Resistance has a lot more to do with the new movies. This year the 10K merchandise was focused on Rey, Finn, and BB-8. Aside from Obi-Wan Kenobi, the
Disneyland Star Wars Half weekend has now honored every prominent hero from the original trilogy. Chewbacca was the theme of the Half medal in 2015 with Yoda on one side of the Rebel Challenge medal, Luke in 2016 with R2-D2 on one side of the Rebel Challenge medal, Han and Leia in 2017 with C-3PO on the Rebel Challenge medal. I think the slight change just allows them to bring Rey, Finn, Poe Dameron, and more into merchandise, medals, etc.
Dark Side can keep its name and theme since the new films refer to the Dark Side. Could the medals change? Absolutely. Should they change? There's an argument for that. I'm not saying they get a radical overhaul every single year, but if they want to keep previous runners interested in coming back, new medals can help that. In other words, the Kessel Run stays, but maybe the Millenium Falcon changes from year to year. Still the Falcon, but not the same one.
Look at what happened when they revealed the new Tinker Bell medals.