There will be no buff for Roshi nor there will be any nerf for swordsman. The damage formula it's possible to be modified in future patches I don't know that. But the most probable reason that roshi has more survivability is that the power levels grow and it will be harder to one shot people (an intuitive reason for this is that out of the primary stats 2 are offensive and 3 are defensive and if you grow them at almost the same rate it's clear that survivability will be favoured to fast killing)