The difference is whether or not you care about sapience as instrumental or terminal values.
If I only instrumentally value other sapient beings existing, then of course, I don't care whether or not they exist after I die. (They will cease to add to my utility function, through no fault of their own.)
But if I value the existence of sapient beings as a terminal value, then why would it matter if I am dead or alive?
So, if I only value sapience because, say, other sapient beings existing makes life easier than it would be if I was the only one, then of course I don't care whether or not they exist after I die. But if I just think that a universe with sapient beings is better than one without because I value the existence of sapience, then that's that.
Which is not to deny the instrumental value of other sapient beings existing. Something can have instrumental value and also be a terminal value.
(Playing devil's advocate) Once you're dead, there's no way you can feel good about sapient life existing. So if I toss a coin 1 second after your death and push the red button causing a nuclear apocalypse iff it comes up heads, you won't be able to feel sorrow in that case. You can certainly be sad before you die about me throwing the coin (if you know I'll do that), but once you're dead, there's just no way you could be happy or sad about anything.
More than once, I've had a conversation roughly similar to the following:
Me: "I want to live forever, of course; but even if I don't, I'd still like for some sort of sapience to keep on living."
Someone else: "Yeah, so? You'll be dead, so how/why should you care?"
I've tried describing how it's the me-of-the-present who's caring about which sort of future comes to pass, but I haven't been able to do so in a way that doesn't fall flat. Might you have any thoughts on how to better frame this idea?