First of all, there is absolutely nothing prima facie wrong with any politician's being challenged in a primary at any time in any place. God knows, the Democrats could use a couple of dozen more good solid primary fights. You can't moan endlessly about how Americans take no interest in government and then complain when they actually do.
I get why politicians have a sad when their buds might lose their jobs, and I get why they hate serious primary challengers, but this "democracy is awesome in theory but horrible in practice" is so often on display from our newspaper editorial boards. The near certainty of incumbent re-election is the worst thing ever in the abstract. But when there's an attempt to unseat anyone with the only method with any chance to succeed for most incumbents - primaries - the fainting couches come out about horrible extremist voters daring to turn their backs on all that experience blah blah blah.