$40 does seem a bit steep for a hunk of injection molded plastic... But it is what it is. If you don't like the price, or the product, you don't have to buy it.
Comp-Tac makes a kydex single AR mag pouch that costs roughly the same as these, last I checked. I own one. The RCS clip might last longer than the built in clip on the Comp-Tac pouch, if you are taking it off and putting it on by slipping it over the belt, vs threading the belt through it. Over the years, I've had a LOT of Comp-Tac mag pouches crack at the bend at the top, where the plastic has to flex. I thread those pouches onto the belt as much as possible now. I have had kydex holsters and pouches crack quite a few times. Comp-Tac used to use bent kydex for their holster belt clips, but eventually started using their current molded plastic clips, as the original kydex ones cracked quite often. The new clips are almost indestructible in regular use.
I was sad when RCS went away from kydex, and started up the molded stuff,because it just looks/feels cheaper, like Fobus stuff. But it honestly makes a lot of sense for anything that needs to flex. Usually its a design issue. Anything made of a hard plastic that needs to flex, is going to crack eventually.
Maybe those old geezers who used leather were on to something...
ETA: If these were priced closer to $20, I'd probably buy one or two to try... At $40, I'm not really interested. RCS does occasionally have some decent sales/discounts, so it's worth watching, IMHO.