For selfish reasons, if I had a say in policy I would want to influence the world greatly against this. Whether true or not, I could easily get a disease in the future or go senile (actually quite likely) to such an extent that my moral worth in this system is reduced greatly. Since I still want to be looked after when that happens, I would never support this.

This doesn't refute any of the arguments, but for those who have some percentage chance of losing a lot of brain capacity in the future without outright dying (i.e probably most of us) it may be a reason to argue against this idea anyway.