Unclear on your question about updating all RAW images without selecting them. No, you have to select them, or use the other setting which will update thumbnails as it goes. In your case, selecting a bunch of images and building thumbnails is likely to work best for you.
As far as prompting people if the thumbnails are low quality, yes, I suppose so. I tend to dislike mysterious-sounding dialogs or having the app behave in different ways based on the image type (e.g., it could see you have crappy thumbnails and then update them) because people won’t understand why some images update and others don’t. And no one reads alerts.