From what I've heard, Stretch uptake is pretty good in some markets and no so much in some others. What makes it tough to judge compared to the old YX Signature are two key differences:

(a) Stretch cannot be pre-reserved by anybody before check-in unless one purchases the Classic Plus fare, or one is a Summit member...but even Summit members may have to call to be upgraded ahead of time because you can't change seat assignment online until check in. So at 24 hours ahead of departure, Stretch almost always looks largely bare. (On Midwest, people could upgrade from any fare, at or anytime after purchase.)

(b) Frontier's seat maps disappear 2:00 ahead of scheduled departure. A fair number of people will have checked in online by T - 2:01, but quite a few have not. Once T - 2:00 comes, you can't tell what happens. Midwest's seat maps showed right up to departure time, so around 30-40 min ahead of departure...before people were bumped up to Signature for free if they didn't have a seat assignment in Saver...you could get a good idea of how many people upgraded.

A key thing to remember about Stretch...as opposed to Signature or conventional first or business class....is that origin and purpose is a bit different, and as such I think the goal differs, too.

Stretch key purposes include:

--Generating upgrade fees
--Rewarding frequent flyers
--Encouraging purchase of bundled fares

Frontier created Stretch without any reduction in seating, and so the economics are a bit different. If Stretch didn't generate a cent of paid upgrade, it would still improve frequent flyer loyalty and encourage the purchase of bundled fares without much of a downside.

Compare this to conventional business or first class, which means fewer seats onboard. The lost revenue from fewer seats creates an immediate deficit that must be overcome with higher revenue.

Although I'm sure Frontier would be disappointed if the uptake on Stretch was 0%, there's no lost revenue potential to recover. It's kind of like charging for exit row, which some airlines do. It doesn't cost anything to do so (other than administrative costs) so whatever revenue they get is pretty much gravy.

As for the pricing, the new fees seem to be aligned pretty close to what United charges for Economy Plus upgrade, which is a fairly similar product.