Nah. Wikipedia is popular because it is the #1 search result for a lot of stuff. Most of people going there just want to look up something for a homework assignment, online argument, or whatever. If Grokipedia has an error rate 5%, compared to 1% for Wikipedia, it's probably still fine.
If Wikipedia traffic shrinks down just to the true "encyclopedia reader" crowd, they will be in trouble, because I suspect that's less than 10% of their current donations. And Grokipedia is already starting to crop up in search results.
Wikipedia has an endowment big enough to sustain the site's basic maintenance, essentially forever. If donations disappeared then they would have to severely cut spending, however, I don't think it would be an existential threat.