In addition to the district's heavily Republican lean, Smith's willingness to break the party line has helped him stay entrenched. Although he is extremely conservative on social issues, he has taken stances that go against party orthodoxy on issues that may be beneficial to his constituents. It wouldn't surprise me if Smith became a lifer.
Notably, he was one of (I think) just four Republicans to vote against both the Trump Tax Cut and the 2017 GOP healthcare bill. The other three (Leonard Lance, Frank LoBiondo, and Dan Donovan) no longer serve in Congress but Smith, unlike those three, was never considered to be in danger of losing re-election.
There are some R+8ish seats that could plausibly flip (IN-5, MO-2, PA-10, GA-7, a bunch of seats in TX) but Smith's seat isn't one of them.
