What's your take on this argument:

"Why do we need longtermism? Let's just do the usual approach of evaluating interventions based on their expected marginal utility per dollar. If the best interventions turn out to be aimed at the short-term or long-term, who cares?"

Coming from an economics background, here's how to persuade me of longtermism:

Set up a social planner problem with infinite generations and solve for the optimal allocation in each period. Do three cases:

• A planner with nonzero time preference and perfect information
• A (longtermist) planner with zero time preference and perfect information
• A planner with zero time preference and imperfect information

Would the third planner ignore the utility of all generations less than 1000 years in the future? If so, then you've proved strong longtermism.

the long-term effects of these actions probably dominate. But we don’t know what the long-term effects of many interventions are [...]

To me, it makes more sense, even if you’re focused on traditionally near-termist causes like mental health, animal welfare, and global poverty, to evaluate interventions based on their long-term effects.

This just seems like a nonstarter. If our estimates of long-term effects are massively uncertain, how can they possibly be action-guiding?

Or, (3'): if we can't calculate $V_t$ for $A_i$ and $A_j$, then assume that they're equal, and rank them by using their expected value over periods before $t$. 

Is this longtermism?

1. List all possible actions {$A_1$,..,$A_K$}.
2. For each action $A_j$, calculate expected value $V_t\left(A_j\right)$ over t=1:$\infty$, using the social welfare function.
3. If we can't calculate $V_t$ for some t, due to cluelessness, then skip over that action.
4. Out of the remaining actions, choose the action with the highest expected value.

So longtermism is not a general decision theory, and is only meant to be applied narrowly?

Is that idea that once these longtermist interventions are fully-funded (diminishing returns), then we start looking at shortterm interventions?

perhaps we'd do better to focus on different interventions: ones whose effects of the further future are more predictable

What's the decision theory here? 

Consider a two-action, two-period model: we know the effect of action A1 in t1, but not in t2; but we know effect of A2 in both periods. Is the suggestion to do A2 (rather than A1) because we have more information on the effect of A2?

Also, this seems like a bad decision theory. I can't estimate the longterm effects of eating an apple, but that doesn't imply that I should starve due to indecision.

Isn't Response 5 (go longtermist) really a subset of Response 4 (Ignore things that we can't even estimate)? It proposes to ignore shorttermist interventions, because we can't estimate their effects.