When TIME Magazine asked this exact question to Wikipedia founder Jimmy Wales, here’s what he said:
While we’ve always said that we’re opposed to having advertising on Wikipedia, we’ve never said “absolutely never.”
According to Wales, if they’re unable to raise sufficient funds through donations in future, they’ll first look at cost-cutting measures and then may include ads in “some obscure locations” to keep Wikipedia alive. That’s very unlikely though!
Here’s the full TIME interview with Jimmy Wales: