I don't think it's selfishness so much as fear of her own "mortality". She does get KILLED in the Sepulcher quests. She's terrified of dying again (she's sort of gone through it a few times). And to be honest, that makes her interesting. She IS interested in preserving the undead she leads. They are her army, (and protection) after all. But her own personal lack of TRUE immortality is enough to terrify her and have her make endless interesting choices.
I like that Nathanos is her fuckboi, that being said, it pits her and Genn at each other's throats and ALSO makes HIM make interesting choices (why isn't he doing his job as general? He's out hunting Sylvannas just for his own personal vengance!)
But if she had succeeded in securing immortiality for herself, do you think she'd deny it to the other undead? I don't.
And I should say, I am a Sylvannas loyalist. And play a TROLL. She's the most interesting character in the Horde. Her personal quest for immortality is fine, and interesting if she makes it. Because if she makes it...well...what WILL she do to the horde?
Because I'm fasinated with the prospect of what the horde will look like when Sylvannas finally can't be killed.
And let's put it this way: She isn't crazy. She didn't get mess with an old god. She isn't bent on world-destruction. She's got more to her than that.