Well it's a bit harder to kill either of them without ramifications. I think in terms of title sequence cast he's only permanently killed off around four of them while they were still in regulars credits. (Doyle, Tara by technicality, Penny and of the big two in Serenity, one was already written out off screen. Everyone else that died generally came back in some form.)
Dawn being the sister and Xander the only consistent male makes me think it'd be a bit excessively huge for them to deal with the ramifications if either died. Plus they've both had pretty unfortunate romantic histories so maybe they cancel each other out, aside from the Buffy tension.