I fly out of SLC frequently and often wish DL had more regional destinations. But for the next few years - until the new terminal is fully operational - they are gate constrained. Capacity and passenger traffic are currently on par with 2019 levels overall, but with gate space being constrained they just can’t add a whole bunch of flights on smaller aircraft. The air travel market to SLC is stronger than most right now, so they are prioritizing flights that use larger aircraft to capture as much business as possible. That makes more sense than trying to serve markets like Moab, which don’t have enough traffic to justify larger aircraft. And the population density of the region makes it impossible to properly compare to the southeast.
While I agree that each of the big 3 airlines has a significant gap in their network, industry wide there are probably too many hubs in the US, not too few. Even with lots of hubs closing or being scaled back considerably over the past twenty years there are probably more than needed to ensure good connectivity between all city pairs. That’s partly because we have a competitive system and each large airline wants the opportunity to move people between all the combinations, but also the result of the fact that there used to be a lot more smaller carriers than there are now,. As a passenger I would love to be able to fly my preferred airline between any two cities globally, but I certainly do not want a system where there is only one air carrier or even just one alliance. Passenger service would likely be far worse than it is now (and yes, I do think that is actually possible, despite how lacking I consider current service levels).