Re: "you'll start seeing a new user interface on eligible Windows 10 devices soon"
"I can't for the life of me understand why dumping users on a continuous learning curve is considered good practice "
It might be a career necessity. If a developer, manager or team doesn't have something like this to show and justify their existence they might be at risk of getting dumped in the next round of reorganisations or other euphemisms.
Useful stuff such as bug fixes is much less attention grabbing.
Alternatively it could just be a sign marketing is running the business.