This is a good point; however, I would also like to point out that it could be the case that a majority of “dedicated donors” don’t end up taking the pledge, without this becoming a norm. The norm instead could be “each individual should think through themselves, giving their own unique situations, whether or not taking the pledge is likely to be valuable,” which could lead to a situation where “dedicated donors” tend not to take the pledge, but not necessarily to a situation where, if you are a “dedicated donor,” you are expected not to take the pledge.
(I am highly uncertain as to whether or not this is how norms work; that is to say, whether norms connecting a group of people and a certain action could refrain from developing even though a majority of that group of people take that action.)
I think one important reason for optimism that you didn’t explicitly mention is the expanding circle of moral concern, a la Peter Singer. Sure, people’s behaviors are strongly influenced by laziness/convenience/self-interest, but they are also influenced by their own ethical principles, which in a society-wide sense have generally grown better and more sophisticated over time. For the two examples that you give, factory farming and slavery, your view seems to be that (and correct me if I’m wrong) in the future, people will look for more efficient ways to extract food/labor, and those more efficient ways will happen to involve less suffering; therefore, suffering will decrease in the future. In my head it’s the other way around: people are first motivated by their moral concerns, which may then spur them to find efficient technological solutions to these problems. For example, I don’t think the cultured meat movement has its roots in trying to find a more cost-effective way to make meat; I think it started off with people genuinely concerned about the suffering of factory-farmed animals. Same with the abolitionist movement to abolish slavery in the US; I don’t think industrialization had as much to do with it as people’s changing views on ethics.
We reach the same conclusion – that the future is likely to be good – but I think for slightly different reasons.