They way they DO stay online for a resilient amount of time is by generating money, i.e. through donations.
In fact, they used to say exactly how many months they have left. Now they don't say that, because they feel they can allow themselves that. Does that mean that it makes sense to stop fundraising? No. Because the minute the public doesn't feel that there's an urgency in donating to your cause, then it's someone else's problem, meaning it's nobody's problem and that's how you kill NPOs.
Money generates money. To fund something in perpetuity you need enough that the returns from investing it (taking into account tax, costs, inflation, etc) exceed the annual payment needed. This is not an infinite amount.
I haven't checked the figures but the article claims the foundation has $400 million cash and Wikipedia costs under $2 million per annum. There's no easy way of calculating the maximum annual sum that can be taken out in perpetuity from a well managed endowment but it's certainly a lot more than 0.5%.