From financial point of view it would be more cost effective to buy a single EV3 brick. However these are rarely sold alone and the price for extra parts could be negligible if bought as a set. It might be easier to find a good deal for a set as well.
It is hard to say if LEGO is looking to release EV3 successor. However LEGO has released a new series of powered electronics as they call them "Power Functions 2" or "PowerUp" with pretty much same specs as previous series, but with new type of connector. This connector is very close to one used with EV3, but different. The whole range is designed to be used with basic robotics system (Boost) as well as general consumer sets (Trains), basically making it one-fits-all. So there is some hope we might see proper robotics system in not so far future.