I have written a couple of times about my feelings about taking the GWWC pledge. (I don’t believe I’m signed up on the website, but I in fact have given 10% for my entire working life, except two years that were particularly bad financially.) I think the essence, for me, is a sense of empowerment.
The world is full of enormous problems that I can’t do anything about. I often feel weak and powerless and helpless. GWWC says to me that I do have power to positively affect the world, and always will. I don’t have to be exceptional: as long as I have money, I can donate to highly effective charities and save lives. I don’t have to worry about dying and leaving the world the same as it was when I entered it. Though I will never know their names, there are people who would be dead if not for me, and who are alive. And that means so much.
I don’t understand why you’re assuming that biodiversity is bad for wild-animal welfare. Biodiversity and population size are conceptually distinct: you can have a population of a single species that is much larger than the total population of five different species. Indeed, the most biodiverse regions (such as the Amazon) get that way not just because they’re very productive but because they have numerous low-population species. Biodiversity loss is disproportionately concentrated among low-population species, as they are the most likely to go extinct.
Similarly, while I haven’t been following the field closely for many years, my understanding is that we are certainly reducing arthropod biodiversity, but may or may not be reducing populations—a 75% reduction in biodiversity is absolutely not the same thing as a 75% reduction in insect populations!
I think (assuming net suffering in nature) biodiversity is probably neutral for animal well-being, and potentially valuable for other reasons such as scientific research.