Matt Levine is _funny_, which is a rare talent; to combine that talent with expertise and desire-to-write is even rarer.
Also, people like to read about money stuff because even if your mammal brain knows you'll never get rich using anything he tells you, your reptile brain still thinks you might.
A similar market niche: home repairs.
Gwern says Greco-Roman philosophy wouldn't work as a niche; but my one offering to the list of Matt Levine simulacra actually does write about Greek and Roman _stuff_, if rarely philosophy: Bret Devereaux at https://acoup.blog . You might know him from his "historian's analysis of the Battle of Helm's Deep" or his three-parter on war elephants. Even though the people being studied are long dead, the people _doing_ the studying are very busy producing new works — there's no danger that ACOUP will ever run out of new books to review or new theories to evaluate.
It's not a newsletter, but it still hits the spaced repetition point. It doesn't have the advantage of following news, but it makes up for this by having a huge 'backlog' of things to write about. And it still has the 'known outcomes' point, because largely the outcome of history is known. And on the open questions, there isn't much progress, but there are themes to hearken back to. So even there you still have spaced repetition.
Finance and the stock market are also, fundamentally, just basic math behind a ton of obfuscated terms.
A lot of scientific fields are most definitely not based in such simple foundations, there will be complicated chemistry at a minimum. Engineering and physics quickly careens into calculus at a minimum.
A similar market niche: home repairs.
Gwern says Greco-Roman philosophy wouldn't work as a niche; but my one offering to the list of Matt Levine simulacra actually does write about Greek and Roman _stuff_, if rarely philosophy: Bret Devereaux at https://acoup.blog . You might know him from his "historian's analysis of the Battle of Helm's Deep" or his three-parter on war elephants. Even though the people being studied are long dead, the people _doing_ the studying are very busy producing new works — there's no danger that ACOUP will ever run out of new books to review or new theories to evaluate.