One thing that's interesting from this discussion is that LAX-DEN doesn't permit connections to other cities either like no TYS-DEN-LAX.

SAN also is like LAX, so some flights like MDT-DEN-SAN are not possible (well technically, MDT-DEN-SAN is only possible on Wednesdays). SAN-DEN-TYS isn't possible.

TYS, MDT, GSO, out east of Texas are markets without Southwest or another LCC, so Frontier kind of fills a role as an only low cost carrier providing low fare access to the West. I'm not sure if the new Frontier wants to play that role, as it migrates to ULCC for larger markets and less connection dependent traffic.

In the meantime, I suppose SNA is there however for those that want to fly from these smaller cities to So. Cal for a low fare flight (likely lower than US, DL, AA) via Frontier.

LAS, SFO, SNA and SEA are still generally possible for west coast connections. These cover the four largest destination markets possibly out west with SNA in between LAX and SAN.

Even as Frontier migrates away connections, I'd be pretty surprised if LAS-DEN frequencies were cut in such a way that spoke city-DEN-LAS, LAS-DEN-spoke city round-trip wasn't permitted or if it was reduced so much that it was only available once a week.