IOTL? He's 61 years old. The average American white male expires at 76. https://www.worldlifeexpectancy.com/usa/life-expectancy-white-male
So, expect the estate to sell the rights in 2035.
I would not expect that to happen precisely in 2035 which is 16 years from now since Watterson could die either sooner or later but it's unpredictable and even then when he actually dies in the future could his estate if one ever does exist be even allowed to sell the rights (It depends on is specified in Watterson's will if one exists) as it would depend on who inherits the rights when he does die.
It is a possibility that the rights might never get sold or it could be his family selling the rights in the future.