OP, I can see where you're coming from, but what Google implemented makes 100% sense, and your suggestion makes only 50% sense.
If you want to secure your phone for privacy purposes, it's best to secure the whole phone.
The idea that you keep the phone itself unlocked all the time but then just don't want thumbnails of apps displayed is not good security/privacy implementation. It is not based on a typical use case scenario. Is your scenario valid? For you, yes. Does that mean that Google isn't doing its job for everyone else? No.