WOTC said from the beginning that monster races are not meant to be balanced. Considering that Yuan-Ti are conventionally immortal snake men hailing from most settings oldest human civilizations that rings doubly true. There is nothing wrong with substituting magic resistance for shapechanger, they are almost equivalent as powerful abilities go (I would give the edge to magic resistance) and I actually really like the flavor.

1) You could have the snake shapechange work like a druid's wild shape but only allow the Yuan-Ti to take the forms of specific snakes (DM's choice).