Theyve had successful donation campaigns so far. Until they fail to cover costs its pointless to assume they cant. Considering wikipedia is one of the most popular websites in the world and a significant cultural phenomenon id say likelihood of that happening is close to 0.
Wikipedia will shut down only when people decide there is no more need for wikipedia and that there are more efficient ways of spreading public knowledge.
We know in Web 2.0 there is nothing ultimately free
Sure there is. For the reader Wikipedia is completely free.