People can learn. It's not like they would never be able to use their computers again. They'd call their daughter and she'd be like "oh, click on that little arrow and click on 'yahoo'".
Yeah, if any major service that millions of people used every day disappeared, there would be a period of adjustment. But that's all... a period of adjustment.
Real World Example: Some relative of my girlfriend set up her mom's computer to use Yahoo. But she has since been convinced that Google is better and she should use that. So what does she do? Every time she starts up her web browser, she types "google" into the Yahoo search box, clicks the link to go to the Google homepage, then does her search there.