In general, I think it's more sustainable to use leather than plastic-based alternatives. In my experience, leather lasts longer and can be refurbished and repaired (especially Hermes leather!). I used to work in the energy space, and the process of making fake leather / 'vegan' leather is not pretty / great for the environment. In terms of it being part of an animal, to me, it's like eating meat: I stopped buying any meat unless I know it came from well-treated animals. As a result I am ok using Hermes' leather because I am reasonably confident that the animals were well treated.
(I have never given much thought to reptile skins as being an unsound practice and need to research this (intuitively, I am more prone to feel bad about a cute calf than a monitor lizard!). )
I haven't bought a non-Hermes bag in a long time, so I feel like I am ok in terms of my sustainability / ethics karma. Most of my leather shoe purchases are Chanel flats, and I am slightly terrified to look up their practices in case they are bad (I really love those flats...).
Something that makes me feel good about Hermes is the idea of artisanship and paying people a living wage to create beautiful, high-quality items. We live in an era where people have SO MUCH stuff; consumerism is everything. I know people who buy something for $10 at H&M, wear it twice and throw it away. It really, really bothers me. And yes, I do understand that Hermes is not exactly an alternative to H&M, and many people can only afford fast fashion. But we need to shift the conversation to buying fewer, higher-quality things, and it's great to see brands putting that idea into practice.
As for animal testing, I don't know how to think about it. I have seen videos against it, and it's terrifying. If there are viable alternatives, companies should use them. But I also think of all the pharma development and other important work that takes place using lab mice (rats?), and how a lot of the scientific advancement would have been impossible without that. Of course, developing life-saving medication is not the same as developing moisturizer, so that's something to think about.