[...] you would have to argue that:
There are longtermist organizations that are currently funding-constrained,
Such that more funding would enable them to do more or better work,
And this funding can’t be met by existing large EA philanthropists.
It’s not clear to me that any of these points are true.
It seems to me that those points might currently be true of Rethink Priorities. See these relevant paragraphs from this recent EA Forum post on their 2021 Impact and 2022 Strategy:
If better funded, we would be able to do more high-quality work and employ more talented researchers than we otherwise would.
Currently, our goal is to raise $5,435,000 by the end of 2022 [...]. However, we believe that if we were maximally ambitious and expanded as much as is feasible, we could effectively spend the funds if we raised up to $12,900,000 in 2022.
Not all of this is for their longtermist work, but it seems that they plan to spend at least 26% of additional funding on longtermism in 2022-2023 (if they succeed at raising at least $5,435,000), and up to about 41% if they raise $12,900,000.
It seems that they aren’t being funded as much as they’d like to be by large donors. In the comments of that post, RP’s Director of Development said that there have been several instances in which major grantmakers gave them only 25%-50% of the amount of funding they requested. Also, Linch, on Facebook, asked EAs considering donating this year to donate to Rethink Priorities. So I think there’s good evidence that all of those points you mentioned are currently true.
That being said, great funding opportunities like this can disappear very quickly, if a major grantmaker changes their mind.
I’m in a pretty similar situation (I’m a CS senior in the US and took a SWE job offer from a unicorn). If I had the choice, I would go into direct work. Ben Todd said the following earlier this year:
To me, those numbers sound high enough to swamp the downsides of working on community building that you mentioned, if you’d like to choose whichever option is higher-impact.
(I’m posting this as a comment because it’s not as well-thought-out as the answers others have given.)