Ok so they're a fairly large organization. Compared to other similarly sized tech companies, they pay pretty far below market rates. They're not exactly looting the coffers.
They are doing pretty well financially, yes, and they have done well with their endowment, but it's not like they have an order of magnitude more money then they are letting on.
Are their banners obnoxious? Yes. Are they effective? Apparently.
What would you rather have? Do you really want Wikipedia to be destitute? "Hey guys Jimmy here we don't really need any money but I guess you can give us some?"
No one knows what the future holds. A strong endowment fund today isn't a guarantee for tomorrow.
I'm glad they're striking when the iron is hot. I hope they build up enough wealth to operate for generations upon generations. Of the things on the modern Internet that offend my senses, the fundraising banners on Wikipedia probably rank dead last.
I don't get how people can be so negative about such an overwhelmingly positive contributor to society.
edit: you know what? Fuck it: I hope they make far more than enough money to pay everyone who works there an incredibly comfortable salary so they can all focus in improving humanity's knowledge. You just convinced me to donate.