I’ve worked a lot with nonprofits. Having a pool of static funds available is not indicative of sustainability. If you are asking for funds only when you are running out, it is far too late for you.
If they cared about long term sustainability they wouldn't use that money to fund a web of charities. If they were expecting hardships in the distant future you would obviously expect them to cut down on unnecessary spending to focus on keeping the website up and running. Is that happening?
Money is fuel. It should not ever become such a constraining resource and saying they can get by with less is trivializing the way they conduct operations and the impact the are obligated to peruse.
I don’t see this argument being made as it relates to for-profit companies. Is a nonprofit unworthy of fundraising because they have “enough” already? Who defines that? Are they not worthy because they bring value to society rather than individual shareholders? Nonprofits still operate as businesses and they also want to see growth. I’m not sure how any reasonable person would see a problem with that.