We should have a button in the sync options to not sync "delete" actions with local storage. Otherwise, we lose the concept of a true archive. This is not saving time or storage but actually creating the need for more (a second copy of my DB) on my hard drive.
So you want to delete photos from Lightroom CC but not delete them from the hard drive? What do you intend to later do with these deleted local copies? If you still want them, why would delete them from Lightroom?
I have a couple of ideas of solutions for you, but I need to understand a little more.