The interesting thing is that we're dealing with two forces colliding in order to force Lynn to simply banish Dirk, Molly and Gayle to the small town that all the other characters that didn't last go. The first such force is Lynn's substitution of repetition for character development. This is why Elly never seems to learn that she's chasing a mirage that's ten pounds lighter. It's why John never learns that Elly feels unappreciated and anonymous. It's why Mike never learns that other people have feelings too. It's why Liz spends her life never really figuring out what love is and it's why April is probably still at war with Becky. Lynn loves to repeat herself....unless said repetition gets boring.
This second said force is why sitcom antagonists vanish over time. Lynn might have wanted to continue with the "Why is she dating this ugly boy when she knows that ugly people should die alone and shunned and unloved?" show but it got to be too boring to look at. It's why Janice got nerfed and became a girly-girl and it's why Mrs Dingle learned to love Mike and Weed.