I doubt money had anything to do with it; at least there's no evidence for it.
What's most likely is this:
a. Magnus is very pragmatic. He knows he can win tiebreaks, so why risk chess's most prestigious title? He's playing it safe.
b. He does seem a little off his game lately, and certainly tired. I mean, he's not even playing in the SCC this year. Maybe he just needs a break.
Not like Fabi would win this match anyway. Magnus got through it without spending all that much effort. It's kind of a disappointing match, but he's not an entertainer. He's a cold, calculating strategist who knows he is not doing as good as he should be. After the first few games, it makes sense that he wouldn't want to expend a lot of effort or risk losing. Fabi is really hard to beat in classical. 2017 or 2016 Magnus would do it, sure. But right now it's just not worth it. Completely understandable.
I wonder if money had anything to do with Magnus’s decision? Would a tiebreak meaning an extension of the World Championship match mean more money for the contestants or chess in general? Would chess being in the spotlight a little longer add to chess? Maybe Magnus was ordered by someone to take a “dive”? If so, Caruna would be forced to not win the title by the same person because to accept a draw, Magnus would need a guarantee to win later on.
Or I am just being waaaay too paranoiac ...