The other thing is, happy characters (or at least their relationships) tend to be put on a bus, unless they're needed to advance another plot line, or to play something for laughs (i.e., dancing while only wearing a puce condom). A happy relationship with nothing dramatic happening isn't that INTERESTING once it starts. That's probably a lot of why we don't see much of, say, Steve and Cossette - there's nothing interesting (read: dramatic or funny) happening in their relationship now. But, we saw them in the lake house scene because they were both needed to advance Claire's character in a funny way.
QC is a large enough comic by now that it wouldn't have to end just because Marten and Claire got a "happily ever after" ending, though. There's still a lot of plot lines there - Dora and Tai, Faye and Angus (which isn't completely resolved), Veronica, Marigold and Dale, Sven has a lot of plot development left, Hannelore has a TON of plot development left (and it's been damn near stated that she wants to have a functional romantic and sexual relationship with someone, which would take a whole lot of time to resolve). Plus, even with a "happily ever after" ending for a hypothetical romantic relationship between Marten and Claire, there's still interesting dynamics that each of those characters could have in the story (I mean, the classic dynamic in the comic is the Marten and Faye dynamic, even when it's just platonic). Not to mention, QC could go to gag-a-day Pintsize comics and still maintain a huge reader base. (That said, I highly doubt Marten and Claire will even enter a romantic relationship, let alone have a happily ever after ending. Just saying that them having such an ending wouldn't end the comic.)
And, I should've said, I don't believe that's the FULL reason her face turned beet red.