The six month expiration policy is quite insane in terms of trying to giving anyone a reason to fly with them.
I flew with them in July 2015 and close to the expiration, I was able to get a $25 gift card for Walmart. I lost about 93 miles. Other members of my family have not been so lucky and now all of them have been zeroed out.
So ... with no miles in the bank, what reason do I have to fly with Frontier? I would need to be going to a city they serve but WN/AA/UA does not. Maybe Frontier would have a time that just happened to suit my needs better.
But, if it is heads-up competition against Southwest, how could Frontier possibly make me want to fly with them instead of Southwest? Price? Unlikely. Sure, they could give me a $29 fare, but I'd want "the works" and then the price would likely be at/near Southwest. They certainly can't get me with the promise of a future free flight given how they expire miles.
Now, you might say "all airlines do this." Yes and no. United and American both offer dining for miles. I haven't stepped foot on a UA/AA flight since 2013 / 2006 respectively, yet I have never lost a mile with either of them. Just go dining at one of the locations now and then and my miles stay safe. Same for the rest of my family. There is no option to do this with Frontier. Yes, I could get a credit card, the perks of the card are not as good as the perks of a Southwest card.
Yes, I think they want the "loyal" flyer. But if I was running an airline, I'd want all flyers regardless. I wouldn't put together insane policies that drive away customers.