Argument: The money can be spent over a long time and like will be able to be spent.  

The footnote on the main question says:

In total. You can imagine this is a trust that could be spent down today , or over any time period

Likewise @Will Howard🔹 argues that this isn't that significant an additional amount of money anyway:

"$100m in total is not a huge amount (equiv to $5-10m/yr, against a background of ~$200m). I think concern about scaling spending is a bit of a red herring and this could probably be usefully absorbed just by current intervention"

Argument in favor of giving to humans:

Factory farming will stop at some point in this century${}_{80\%}$, while human civilization could stay for a much longer time. So you can push humanity in a slightly better long-term direction by improving the circumstances in the third world, e.g. reducing the chance that some countries will want to acquire nuclear weapons for conflict because of wars because of famines.

So there's an option to affect trajectory change by giving to global health, but not really for animal welfare.

Argument: Approximations are too approximate.

@Henry Howard🔸 argues that much of the scholarship that animal welfare estimates are based on is so wide that that it doesn't make clear conclusions:

Unfortunately these ranges have such wide confidence intervals that, putting aside the question of whether the methodology and ranges are even valid, it doesn't seem to get us any closer to doing the necessary cost-benefit analyses.

Argument: Nietzschean Perfectionism

@Richard Y Chappell🔸 theorises that:

maybe the best things in life—objective goods that only psychologically complex “persons” get to experience—are just more important than creature comforts (even to the point of discounting the significance of agony?). The agony-discounting implication seems implausibly extreme, but I’d give the view a minority seat at the table in my “moral parliament”

To my (Nathan's) ears this is either a discontinuous valuation of pleasure and pain across consciousnesses or one that puts far more value at the higher end. In this way the improvement to the life of a human could be worth infinite insects or some arbitrarily large number.