Change fees and bag fees (or lack thereof) are what keep me over at Southwest. You can get rid of the bag fees at Frontier by either making status (I can't fly that much) or by buying the Classic Fare. If I ever did have to fly on Frontier because a destination wasn't served by Southwest (ANC comes to mind), I'd buy Classic because for about $20 extra, you get two free bags and direct TV. It also reduces the change fee, I think, but not to ZERO.

No change fee is a big deal with me. It makes life so flexible and easy.

IMO and IME open seating improves my chances of an open middle seat, but YMMV.

Depending on your destination and flying patterns, Southwest, even with their changes in Rapid Rewards 2.0, offers more than Frontier.

Consider that at Frontier, a flight from Denver to Las Vegas is 629 miles. You need to do that 25,000 / 629 = 40 one way segments to get a free trip.

At Southwest, under the new program starting March 1, if you pay $80 for each flight, you get 80 x 6 = 480 points for each flight. If you do that 20 times, you have earned 480 x 20 = 9,600 points. Those can be redeemed at 60 points per dollar, which equals $160 of flight credit, or one round trip. For DEN-LAS, that is twice as good as Frontier.

However, it is certainly true that if you are flying longer distances, the edge shifts towards Frontier. DEN-PHL is 1,558 miles and you'd need just over 16 one-ways for a free trip. You'd still need 20 at segments at Southwest.

My opinion: $0 change fees and $0 bag fees. It would give them a fighting chance against Southwest. For destinations not served by Southwest, compare prices to United and set accordingly.