It seems for each generation of airliner, the smallest one from the previous generation is the one that doesn’t do so well.  The 737-500 sold reasonably well but the -600 was the unloved on of the NG range.  The -700 sold well but the Max 7 is not really attracting any love (except from Southwest).  The A320 family has the same thing.  Apparently, the small one just doesn’t age well.  The 737-600 did not end up with many airlines so I have few shots of them.  Aside from the Janets, these are the only operators I have seen.