Of course it will be smaller, however that does mean that tackelling climate change will not make a sizeable contribution towards reducing the risk of nuclear winter. The question for me is whether nuclear winters that relate to climate change are more or less tractable than nuclear winter as a whole. My view would be that trying to reduce the risk of nuclear winter by tackelling climate change and its consequences may be a more tractable problem then doing so by trying to get nuclear weapons states to disarm or otherwise making nuclear war less likely in general, but that efforts to make nuclear winter more survivable are probably more efficient than either of these policies from a purely x-risk reduction perspective.
However, I also do not think that nuclear winter is the only way in which climate change may lead to an existential threat (at least reading existential threat to include the prospect for an unrecoverable from civilisational collapse) as there are some interesting feedback loops between environmental and social collapse that have the potential to cause non-linear and self-perpetuating shifts in the structure of global civilisation. Admittedly these are hard to study, but from a value maximisation perspective I would say that in the face of uncertainty we will do better if we assume that global civilisation is relatively fragile to such changes than if we assume that it is more robust to them.
I think that referring to the successes you mention as ‘peak Quakerism’ actually misses the point rather. At the time of its establishment Quakerism was a very outward looking sect seeking for radical renewal of the world in the aftermath of the English Civil War. At one point perhaps 25% of the population of England were convinced of the truth of the tenets of Quakerism. However, this period was short lived and the movement faltered in size and importance and became highly insular shortly afterwards.
This insularity is not at all unrelated to the successes you talk about. For instance, one of the key aspects of Quaker business successes was that the society (Quakers refer to ourselves as The Religious Society of Friends) provided a very strong social network with a high degree of interpersonal trust. Quakers would know that they could seek one another out wherever they were and arrange business transactions on favourable terms. If anyone was found to have been acting in bad faith they would be ‘cast out’ and shunned by other Quakers, which due to the highly insular nature of the society would generally mean losing contact with all their family and friends and quite likely a good deal of their property as well. There was also a tradition that if a Quaker business were to fail then other Quakers would pay its debts off, which again was generally very good for business. While some Quakers still did act highly dishonourably (e.g. The famous mid-victorian case of Overend, Gurney and Company) these advantages were of significant value, especially to bankers and merchants, and explain lot of Quakers commercial successes.
One can make a similar argument about Quaker opposition to the slave trade. One of the things that caused Quakers to produce some of the earliest corporate statements in opposition to slavery was that slave owning was a huge problem for individual Quakers and their communities. Quakers, especially in the US, were among the most prolific slave owners. However, other Quakers were also convinced that slavery was abhorrent and needed to be abolished. The insular nature of the Society meant that in many meetings in the US these two groups were in close connection with one another and it was very difficult for either group to escape from this connection. This meant that individual meetings had to work to resolve such disagreements if they were going to survive. There were many instances in which meetings resolved them by ejecting those who were opposed to slavery (again have a look at the history of Benjamin Lay, perhaps the most ‘cast out’ Quaker in history). However, within time the dispute was eventually settled largely in favour of the anti-slavers and this is often what lead to these early statements. When the society was finally lead to go further and say that not only was slavery not fit for Quakers but that it should be abolished in society as a whole (this certainly did not happen overnight) it’s dense social networks also provided highly valuable as a means of organising the anti-slavery movement and local meetings became valuable resources.
I could go further, it is a fascinating history. I am a longstanding Quaker and I love the society and our history. However, I do think that a lot of people have a very limited view of how the society operates and why it has had some notable successes. For what it is worth my personal view is that Quakers have a lot to teach those who are willing to patiently come to understand these things but that there are also many many examples of people drawing the wrong conclusions. The Quakers are, and always were, a peculiar people and in many ways the society is a failure. At only 300,000 members worldwide and divided into many factions, most of which do not see eye to eye on a great many things, the society is a long way from peak anything. However, we still have our successes (for instance I was directly involved in successful efforts to use our special marital exemptions to force the UK government’s hand on introducing equal marriage, since we believed that we could go ahead and do it anyway if they didn’t change the law), and I am glad of that.