Copyright doesn't turn public domain if you don't exercise your rights. A work only becomes public domain if the copyright period lapses or (in some countries) if they make an explicit declaration to that effect.
Copyright would cover the original design documents for the sets and props, but not the shapes and colours themselves 'as seen on TV'.
From what I understand unlicensed clean-room reproduction of sets could violate design marks, if those were even registered. Like other trademarks those do indeed need to be enforced.
But trademark infringement doesn't trigger merely from copying.
The infuriating thing about dealing with lawyers on IP is they'll write a C&D that "your work infringes on the IP of my client." There is never any mention of what infringes, except the broadest possible, and there is no responsibility for a false claim.
From what I understand unlicensed clean-room reproduction of sets could violate design marks, if those were even registered. Like other trademarks those do indeed need to be enforced.