I'm not sure exactly what you mean by "main plot", but I'll respond to a couple of possibilities.
If you mean that Marten, who was originally the main character, has morphed into a bit of a Jerry-Seinfeld, nothing-happens-to-me-I'm-just-here-to-quip character, I think I agree with you. It's been a long time since anything remotely interesting has happened to him. On the other hand, given his personality, I'm not sure how much more potential he has. Which leads directly to...
If you mean the Marten-Faye relationship/Marten-Faye-Dora love triangle, then I also agree that it's faded, but I disagree in the sense that I'm glad. That had definately run out of steam, and though it could possibly be interesting again under the right circumstances, said circumstances haven't developed.
This has always been a slice-of-life type story. It's more about the characters than about one sort of "main plot". and if you miss the old love triangle, well, there's a new one for you (Faye-Angus-Sven). Personally, I like the Hannelore-Marigold odd couple friendship stories.
And of course, it's all about taste. Some people may like it better for exactly the reasons you think it's worse.