In order to speak rationally about it, you would have to give the likelihood ratio you believe they could reduce the sum of existential threats by, however low that is; then multiply that by the utility you would receive from saving human civilization. Compare this to the utility you receive from saving a puppy, multiplied by the probability of saving that puppy. Compare the two numbers.
Rationality means thinking quantitatively where applicable, not qualitatively.
Chance that a bunch of nice, sane, ordinary people can save the lives of lots of cute animals (or hell, humans)? Very good.
Chance that a bunch of crazy people with their crazy web site can stop aging, or better yet, stop an asteroid?
Slim to none.