As Ezra Klein says, it’s ultimately far too limiting to look at the question of what incumbent members of congress ought to do purely through a narrow lens of electoral interest. Obviously nobody gets into politics without a certain nose for the electorate. But equally obviously, no congressional majority lasts forever. That’s life. Nobody’s that savvy.
But that’s okay—it’s not the job of an elected official to lurk around in office forever and ever. The job of an elected official is to do things. Hopefully things that make the world a better place. Winning an election is an opportunity. Not an opportunity run for election again—you can run again if you lose—but an opportunity to change the world. Look up any former legislature in the history books or on Wikipedia or what have you and you’ll see that he or she is remembered (or not) for his or her accomplishments (or lack thereof). Everyone leaves congress sooner or later, maybe in defeat or maybe in a coffin, but what ultimately matters is not how long you stay but what you leave behind.