The prospect of including Legends in a dynasty is very intriguing to me. I think it would be a very fun dynasty if one were able to have a hybrid league, in which current players played alongside greats from past decades.
I don't think it would be ideal to have such greats in all created franchises by default, but an option to enable it would certainly open up the game to an extent beyond strictly using classic teams from the past.
Generally, I would say I support this inclusion of Legends in dynasty, but such support is grounded very heavily on the notion that such a feature can be enabled and disabled, and is not defaulted and unchangeable either way.