Actually, there are already quite a few things you can do to hide rows or tables, to restrict editing, etc. One of my docs has about 20 editors, but there are many tables they can see but not edit, and many more tables they don’t see.
It is just not very secure. In normal doc mode, the search option (top left) can’t be hidden and it allows people who use the proper search terms to find just about anything.
For example, you can hide (or filter) a salary table, but searching on an employees name will bring a user eventually to a row with salary information.
Coda is working on ways to make your doc a lot more secure, among other things by syncing pages to other docs without revealing or submitting all underlying source data - but we are not quite there yet.
In this post you can see the current status for more secure sharing of data: Launched: Sync page access control (view-only)