[Linkpost] My doubts about longtermism

Intro/summary:

Will MacAskill, arguably the biggest proponent of longtermism, summarises the argument for it as:
1. Future people count.
2. There could be a lot of them.
3. We can make their lives go better.
On the face of it, this is a convincing argument.
However, this post outlines my objections to it, summarised as:
1. Future people count, but less than present people.
2. There might not be that many future people.
3. We might not be able to help future people much.
To this, I will add a fourth: there are trade-offs from this work.