Thanks for this very interesting post !
I've been thinking a bit about examples of causes and interventions with increasing returns (I'm actually working on a philosophy paper that touches on this issue), and it seems to me that many examples could be found in causes and interventions that involve social norms and politics.
For example, suppose you are putting resources in a campaign to encourage members of Parliament to vote in favor a certain law, which would have a great impact if passed. There may be increasing returns of campaigning at the point where the campaign succeeds in convincing the majority of members of Parliament to vote for the law. This is because there is a threshold: if you do not spend enough resources in the campaign to convince half of the members of Parliament, the law is not passed and your impact is very low; but as soon as you reach the threshold of resources necessary to convince the majority, the law gets passed and you have a very high impact.
The same happens with social norms. Some social norms correspond to equilibria which are hard to modify, so a critical mass of efforts could be necessary to shake them, and then it becomes easy to shift them towards other equilibria. For example, if you want to spread the moral norm of antispeciesism, there might be a critical mass of antispeciesists necessary to make antispeciesism mainstream, and speciesism blameworthy in society. After the critical mass is reached it might become much easier to make progress.