One of the more underappreciated aspects of modern U.S. politics is that the incumbency advantage is a *lot* smaller than it once was say 20 years ago, at least for members of Congress.
This trend gets disguised because districts are a lot more polarized now (and in some cases gerrymandered), which reduces the number of competitive races. But controlling for district partisanship, the incumbency advantage is maybe only half as large as it was during the '90's.
This helps to explain why many, many districts are in play in the House, despite increased polarization. There aren't nearly as many swingy, say, R +3 districts as there once were. But before, a strong R incumbent in that district might have been relatively safe. Now, maybe not.
That's also not to say that some individual incumbents aren't stronger than others. For instance, incumbents who vote against their party more often tend to overperform electorally. But as parties become more parliamentarian, there are fewer of those folks than there once were.
