1. You don’t seem to account for inflation. $10 buys a bed net now. $10 will buy a lot less in 60 years.
2. Helping people accrues its own compound interest: saving someone from death, disability or grief now makes them happier, healthier, more productive members of society, better able to contribute and lift up those around them. Better to have these benefits sooner rather than later.
3. By your same logic, why stop at death? Put it in some trust fund to compound and be distributed in 100, 1000, 1 Billion years

I think you write with too much certainty. You lean heavily on Rethink Priorities' estimates. Sure they've put more effort into estimating bee or shrimp suffering than anyone else has ever bothered to but they are towers of questionable assumptions* that shouldn't give you the confidence to make statements like "If you... restrict your animal-product consumption to large animals, you can easily eliminate 90% or more of the suffering you caused"

Be uncertain. I think if you're honest about the error bars on these analyses, it becomes obvious why people like me bristle at the idea of bulldozing a Chesterton's fence like "factory farming of cows is more important than honey/shrimp"

*e.g. "bees' behaviour tells us something about their capacity for suffering", "death by stinger is painful", "premature bee death means more suffering", "health of bees is a proxy for suffering"... it goes on (a lot)

I think if you see desertification as good (you seem to be saying it is), you should have very high suspicion that your ethical framework has led you astray somewhere.

Taking the most effective actions to help these beings

More targeted interventions directly focused on helping soil life are likely to be far more impactful

Seems like we're far from a consensus even on whether more or fewer of these organisms is the goal. You suggest that biodiversity loss is bad but Vasco Grilo suggests more monoculture farms is better because that leads to fewer microorganisms and he considers their lives net negative.

Give 1000 researchers 1000 years to study nematodes and demodex mites and I don't believe they'll be able to tell you whether their lives are worth living, let alone exactly what interventions would improve them.

A road to nowhere with great reputational cost

my best guess that they have negative lives

Why not advocate for massive desertification efforts and spreading radioactive material to sterilise the soil.? Bring CFCs back to eradicate ozone.
 

same could be said, although to a lesser extent, about caring about invertebrates

Yep agree. Invertebrates is approximately the point on the moral consideration spectrum at which the huge numbers * tiny numbers with highly uncertainty makes the ethics too fuzzy and volatile to be fruitful.
Somewhere between lobsters and maggots the numbers shoot off towards infinities and the whole thing becomes not worth thinking about.

Organisations using Rethink Priorities’ mainline welfare ranges should consider effects on soil nematodes, mites, and springtails.

1. Reductio ad absurdum: If we consider the lives of nematodes and mites meaningful, suddenly all human welfare questions become meaningless compared to the question of how our behaviour affects nematode/mite welfare. The conclusion will be that we either need to nuke ourselves or completely restructure society around maximising nematode wellbeing. This is impractical, and like many internally consistent but impractical philosophies (nihilism, antinatalism, Kaczynskiism) aren't conducive to a functioning society.

2. Poor analysis: The calculations are always the same: huge numbers multiplied by tiny numbers, all of which are highly uncertain and unlikely to become more certain with "more research" (highly doubt any study is going to illuminate the moral value of mite suffering)

3. Looks crazy: Even mentioning the issue to say why it doesn't matter has a significant cost: the fact that it was considered seriously enough to warrant rebuttal makes the organisation look crazy to normal people, in the same way that Rethink Priorities running an analysis on whether nuking Australia would be net good or bad would look crazy.

In practice people aren't able to figure out the the nuances a person's net wealth and income and expenses are and how much of a sacrifice giving 10k really means to them. So they're forced to make a judgement quickly based on limited information.

Being vegan is a less noisy signal of personal commitment than giving 10k to charity, so people will take this more seriously, whether that's fair or not.

I intentionally said "similar", not "the same".

It's slightly easier for Gates to be vegan than me but much much easier for him to give $10k than me

I think you're spot on with the importance of signalling. Personal sacrifice is a strong signal that you believe in something and are serious about it. This is more inspiring and influential to others.

Donating $10,000 to an animal welfare charity is not good proof of personal sacrifice because $10,000 might be basically nothing for a very rich person. Unless people know how rich you are they can't interpret much from this.

Going vegan, however, is a similar level of inconvenience across most wealth levels. Whether you're on Struggle St or you're Bill Gates, giving up eggs and cheese sucks to a similar degree. So when people see that you're vegan they see personal sacrifice and serious commitment.

utilitarianism isn't taken to have a concept of 'permissible'

Don't understand this point. OP is comparing giving $10,000 while killing 2 people to doing neither. Or being vegan to not being vegan while giving $$$ to animal welfare. Clearly by "permissible" they mean "higher utility than the alternative"