WEI does prevent any customization.
Or by isolating the browser from third party software. Android does not let applications mess with each other. Windows already prevents non-elevated applications from touching elevated applications (i.e. running as administrator).
What makes you think that Windows won't add an "untouchable" mode to executables belonging to "approved" browsers? The kernel is already locked down so you won't be able to bypass it that easily.
Android lets applications mess with each other. That's how the accessibility features work.