Trenched? You mean Iron Brigade? Why call it by the old name?
Either way,sure, FXAA probably *would* blur highly detailed sections at native res. (Considering the game's only built in AA is FXAA)
But once the downsampled buffer is resolved, any blur is non existent.
It's no different than using SMAA.
Again, Bionic Commando
1800p with SMAA downsampled http://i3.minus.com/iby4hAFkXFPJKq.png
1800p with FXAA downsampled http://i3.minus.com/ibzOAp2kJ7wKIk.png
There is literally almost 0 difference in IQ/AA in these images other than SMAA handles diagonals better in the form of the power lines in the upper part of the image. (And some difference on the main character)
I'm not sure why you keep cherry picking games with the lowest level of detail. Also, worth noting is that unless you are using GeDoSaTo and specifically using the Bicubic scaling algorithm (the default is bilinear) your results WON'T look the same as your images since displays/GPU's by default scale the image with bilinear scaling, not bicubic like photoshop and other image editing programs tend to do.
Comparatively, here is a game with finer details which while downsampling, using FXAA has blurrier chainmail and loses some of the shine detail as well as blurrier trees in the background, compared to SMAA which has more aliasing yet is also sharper.

