I could have been on-board with your cynicism until the "good riddance" part. Aren't we ashamed that the climate is so divisive that people don't even trust life-saving vaccines? Plenty of the blame goes to people such as OP, really. Sush disdain.
But saying "the more antivax deaths the better" just isn't OK. Those are human beings with parents, kids, siblings, extended families, and co-worker who may have made huge positive contributions to their communities and society.
Dehumanizing people isn't the answer. You can do better.
For example, one might argue that a prominent anti-vaxxer's lies convincing many people not to get the vaccine is extremely harmful; meanwhile the injection of reality by way of that anti-vaxxer's death could reverse some of that harm; and therefore hoping for high-profile anti-vaxxers to die in a high-profile way from the disease whose vaccine they decry is an ethical act.
Squinting a bit, we've just stumbled on something that looks a heck of a lot like a trolley problem! which of course is a famously difficult ethical dilemma with seemingly no good answer---or, at least, no one answer that satisfies everyone. Consequentialists might agree with the grandparent; deontologists would likely agree with you.
The point is: your "shame on you" response is not really engaging with the grandparent's argument, and is presupposing a not-entirely-uncontroversial view of ethics. Again, I'm not claiming right or wrong. The issue is that a "shame on you" argument has the effect, intentional or not, of obscuring your assumptions about the world and demonizing those who disagree with you.
So: perhaps you can do better, too?
Second, I'm fine engaging with the trolley problem on a conceptual / philosophical level. But based on the tone and content, it seemed the poster was happy that antivaxxers are dying: "for every antivax death, we get closer to 100% vaccination rate and finally get back to a normal life."
That's not a philosophical discussion about a real-world version of the trolley problem; that's hoping for more antivaxxer deaths so s/he can be happy.
After reflection on your post, I still think a gentle rebuke for the poster's very real schadenfreude is an appropriate response.
I feel a slight sadness for any early death, but even as a devoutly introverted person, I feel greater sadness for the years of social and family activities and living in fear of other humans. That's its own loss of living, even if it's not a loss of life.
If anyone wants to prove me wrong please do, I find my view of this extremely depressing.