I'll elaborate on why I don't think Saru will be killed off.
1. They didn't know this would be the last season when it was being filmed. If they thought there might've been a Season 6, they would've wanted Doug Jones back. I don't think he would've wanted to leave the show prematurely.
2. Let's go with the idea of "Marriage & Happiness = Death". Okay. Fine. If they're going to choose who they're going to kill off, it's going to be the character who's not in the opening credits. Saru's actor is in the opening credits. T'Rina's isn't. If anyone's going to die in this couple, it'll be T'Rina.
3. If T'Rina had been killed off, and there was a Season 6, the writers could've thought, "Okay. Now we can see how Saru deals with this." They can even do that at the end of Season 5 if, in fact, T'Rina is killed off.
4. I think the fact that some people expect it means it's not going to happen.
5. Rayner won't last as First Officer. Something is up with him. Which will leave the First Officer spot open once again. At the end of the day, this is a TV series. TV series like the Status Quo, which means Saru would be back to his old position by the end. Ergo, he can't be killed off.
6. Finally, Riker and Troi were married in Nemesis. Neither one was killed off, and TPTB still try to have them in as many Star Trek series as possible to this day. Proof right there that marriage doesn't automatically mean death.