They could certainly improve known problem spots e.g. Lyco valve guides but I think they are afraid of admitting that the old design was defective.
IMHO it isn't so much product liability or certification, as the fact that a lot of people would start pressing them for special deals on overhauls of the pre-modification engines. In another business, I would adopt the same cautious attitude, perhaps.
Lyco have finally done away with the really major weakness which was their "disappearing tappets", by bringing in roller tappets. But they have ensured that you need new crankcases to have these, which ensures that it will almost never be an economic retrofit.