Copyright holders are obligated by law to aggressively protect their "works" or risk losing their rights to the public domain. BMI and ASCAP (and others) actively "police" violations of their member's works. Copyrights are "durable property" and pass to heirs upon the death of the copyright holder. Heirs have full renewal rights.

Most British OTR, Including Agatha Christie Mysteries and BBC shows are still zealously protected many decades after originally being broadcast.

Heirs often conditionally release entire series to us in the interest in preservation. If they become stagnant they are of little value to anyone.