A commonly-listed “core idea” of effective altruism is replaceability: it’s okay to take a job in an unethical industry because if you don’t, someone else will, so the counterfactual effect of the unethical industry on the world is the same whether or not you take the job. In practice, the impact of this argument tends to be extremely exaggerated, for at least three reasons.

Threshold hiring

Many firms, including e.g. earning-to-give darling Jane Street, do not hire to fill quotas. They hire as many qualified people as come their way. If you take a job at Jane Street, it won’t cause them to deny someone else a job—they’re expanding as fast as they find people who can do the work. Threshold hiring may not be the most market-rational thing to do (they should be spending more resources on hiring), but:

There’s an extreme shortage of STEM talent, so plausibly Jane Street couldn’t get additional talent even if it raised its pay substantially. Resource allocation is a hard enough problem that we should expect a fair amount of irrationality to persist, especially in industries with high margins (hence less competitive pressure towards purely rational allocation). Even if it is irrational, at the end of the day that’s what they do, so that’s what we should base our decisions on.

If you work at one of these firms, then, in the counterfactual, they actually have fewer resources and can do fewer unethical things. (Note: I separately also strongly believe that Jane Street does not do unethical things, and is hence still a good choice for earning to give.)

Points 1-2 also suggest that threshold hiring may be especially prevalent among firms that are good for earning to give. My experience in software and finance is consistent with this, but also highly anecdotal.

Supply and demand

Even if you work for a firm that doesn’t practice threshold hiring (i.e. has a demand curve, like a rational microeconomic actor), replaceability still doesn’t hold.

Let’s consider the simplest model that actual economists use: there’s a demand curve for labor made of a bunch of firms, and a supply curve made of a bunch of people (i.e. applicants). By applying, you shift the supply curve down. Then the proportion of the gains internalized by producers (applicants) is $\frac{e_d}{e_s+e_d}$ and the proportion internalized by consumers (firms) is $\frac{e_s}{e_s + e_d}$ , where $e_s$ is the elasticity of supply and $e_d$ is the elasticity of demand. If demand is highly elastic (e.g. in “threshold hiring,” talent elasticity of demand is infinite) then producers capture a significant portion or all of the gains in resources that you create by joining the market. Even in many markets that don’t practice threshold hiring, talent elasticity is rather high.

General illegibility

The above model is highly simplified. But even in the more general case, firms’ hiring decisions are highly opaque. The labor market is inefficient due to massive information asymmetries (employees don’t have salary data; firms can’t evaluate employees). Evaluating the counterfactual cleanly and generally, as the linked post attempts to do, is nearly impossible. Replaceability may apply in specific cases, but it depends far, far more on the details than on general considerations of labor movement.

Conclusion

In most practical circumstances, the replaceability consideration fails to hold or is impossible to assess. People choosing their career altruistically should be cautious when applying it, and should under no circumstances use it as a fully (or even reasonably) general counterargument to “ends-justify-the-means” objections to earning to give. There are likely other counterarguments sufficient to show that earning to give is not a net negative, and the general principle of counterfactual reasoning is sound, but the replaceability consideration itself is highly suspect.