Nobody thinks a doman will be the most important thing to control in 2123.
What I would probably do instead is have trust managers or family members or offspring of friends appointed in a legal trust to continue the registration of the domain if it were that important. This frees up the option to move the domain to any registrar based on whatever needs may arise. If the family members lose interest or bad decisions I made in the past taint my legacy then they can stop wasting money on the domain. But that's just my personal preference. I'm sure investors love the idea. Perhaps the low friction option of paying for 100 years and just taking the risk is worth it to some.
As a side note an electric chisel with a tungsten carbide bit and a rock cliff or cave made out of extremely hard rock can persist messages for a very long time. a.k.a. old school blogs. I would wager that someone here could combine that chisel with a frame and some servos to automate carving some beautiful calligraphy into rock walls and caves.