As an egoist myself, the prospect of a very very long life would push me to care less about long term existential risk and care more about increasing the odds of that prospect of a long life for me and mine in particular.
Having no prospect of an increased life span would make me more likely to care about existential risk. If there's not much I can do to increase my lifespan, the question becomes how to spend that time. Spending it saving the world has some appeal, particularly if I can get paid for it.
I think the original post is mistakenly conflating consequentialism and utilitarianism. Consequentialism only indicates you care about consequences - it doesn't indicate whose consequences you care about. It certainly doesn't make you a utilitarian, or a utilitarian for future beings either.
Oh, I wouldn't advise you to do something about existential risks first. But once you're signed up for Cryonics, and do your best to live a healthy, safe, and happy life, the only lever left is a safer society. That means taking care about a range of catastrophic and existential risks.
I agree however that at that point, you hit diminishing returns.
Many people on Less Wrong believe reducing existential risk is one of the most important causes. Most arguments to this effect point out the horrible consequences: everyone now living would die (or face something even worse). The situation becomes even worse if we also consider future generations. Such an argument, as spelt out in Nick Bostrom's latest paper on the topic, for instance, should strike many consequentialists as persuading. But of course, not everyone's a consequentialist, and on other approaches it's far from obvious that existential risk should come out on top. Might it be worth to spend some more time investigating arguments for existential risk reduction that don't presuppose consequentialism? Of course, "non-consequentialism" is a very diverse category, and I'd be surprised if there were a single argument that covered all its members.