A record, not a diagram
People, the households they made, and who came from which. The tree you see is drawn from those three facts every time — nothing is stored as a picture, so nothing can be true on the canvas and false in the record.
Public to read, yours to write
A family marked public can be read by anyone with the link, with no account. Editing it always takes an invitation from whoever owns it.
Nothing is lost by accident
Every change can be undone, deletions say in plain words what they will take with them, and folding a branch away only ever changes what is drawn.