Allegiant has also done Oakland. Not sure if it is still going.

As far as it being a matter of time...okay...sure. 50 years perhaps there is a need for a secondary airport. Maybe.

Atlanta has one airport. Seattle just barely got a second and it only has two gates. These are metro areas much bigger than SLC. And there are many more examples like this.

Salt lake is also expanding. There is almost no incentive for an airline to split its operation- fly to both SLC and Provo, and none of the airlines flying to SLC are going to abandon it for Provo.