I can believe that they would convince her. But steel manning her argument should go no further than having her point out what you just said, and should not include adding unrealistic or unusual evidence to the story.

And that's if it is indeed a strong thing to point out, but even if some ponies will still choose death, alicorns can easily avoid watching them. There's nothing to say an alicorn has to hang out with non-alicorns. (The most deaths an alicorn might see in waiting for their non-alicorn friends and family to die off is probably less than the amount a particularly long-lived human might see, and I've never heard of an centenarian who stopped caring about everything because of witnessing deaths.) If non-alicorns were still choosing death all throughout the future, their deaths could be mere faceless numbers to alicorns that didn't want to think about it. Much like the millions of humans dying today in real life, and the billions who have died throughout history don't emotionally traumatize me.