Saying that extinction has infinity disutility seems reasonable at first, but I think its completely wrong. I also think that you bear the burden of proof if you want to argue that, because our current understanding of physics indicates that humanity will go extinct eventually, and so there will be finitely many humans, and so the utility of humanity is finite.
If you accept that fact that extinction has finite negative utility, it's completely valid to trade off existential risk reduction against other priorities using normal expected value calculations. For example, it might be a good idea to pay $1B a year to reduce existential risk by 0.1% over the next century, but might arguably be a bad idea to destroy society as we know it to prevent extinction in 1000 years.
If you accept that fact that extinction has finite negative utility, it's completely valid to trade off existential risk reduction against other priorities using normal expected value calculations. For example, it might be a good idea to pay $1B a year to reduce existential risk by 0.1% over the next century, but might arguably be a bad idea to destroy society as we know it to prevent extinction in 1000 years.