If that's what happens when you've got hundreds of experts working to prevent it then why do you think it'll be a less of a problem when it's random non-experts?
edit: The imperfection of the current system does not prove that another option is better.
Every major operating system has this regardless of whether they force you to download software through one marketplace. You're not less-secure if you use two marketplaces as long as both those marketplaces are kept secure. iOS is kept secure independently of the App Store as well.
edit: The imperfection of the current system does not prove that another option is better.