To be fair, convenient endings are pretty much Miyazaki's Modus Operandi. And to be honest, I think it works pretty much in all of them. Is it realistic? Hell no. But at least we know why the story did introduce the scarecrow, and it really just brushes off the part of the story that isn't really that relevant - what we're meant to care about in the end is Sophie's and Howl's development and romance, and that certainly shines through in the end, I think.