Its not that hard to understand. Nolan wasnt insulting her, or anything else. What he meant by a “pet” is how pets are to humans. No matter how much you love them, you know you will outlive them, you know you will never truly be attached to them. Nolan does love Debbie, but as he said, she is like a pet to him. This also applies to how Earth is like to him. By his perspective, Nolan isnt an evil character.
“Whats 17 more years?”