It is especially bad writing to have your protagonist win because someone else just makes him win at the end. This is called a deus ex machina, which is fancy-pants talk for "someone or something else just makes your protagonist win". Deus ex machina devices are surprisingly common, in literature high and low, popular and erudite, modern and pre-modern. If your villain suddenly repents, for instance, your protagonist's old friend who has not been part of the story suddenly arrives to rescue him, or a phoenix shows up to save your protagonist by delivering to him a magical hat wrapped around a magical sword in his moment of need, you've got a deus ex machina.
Popularity doesn't make it right. Your hero should succeed because of his own efforts. Anything else is bad storytelling.