Not at all. There are several parameters to look at.
Obviously if you can do it better for cheaper, you have a good chance. But also if you can only either do it better or cheaper you might still have a chance.
Maybe they do something wrong that scares people. Look at Vimeo that came long after YouTube, but Vimeo turned the ownership around not taking it away from the publisher and therefor made it better.
Good way to start is to identify flaws of the established product you have found and find a potential customer or a customer that is not satisfied by how it works now and offer them a good deal for changing to your product, you might even offer them to influence the development of your product. Eventually offer them to transform all their current data into your system for free, that's something companies loves and something that otherwise would scare many off.