The 6N is fine as it is. Its too short a tournament to have bonus points as they would impact quite badly.
As someone said earlier, it would the theoretically possible for a team to get a Grand Slam and still not win the Six Nations; unlikely, but mathematically possible. The most likely combination could be
► The Grand Slam team wins all five of their matches with no 4 try bonus.... 20 pts
► The 6N Winner wins four of their five matches with 4 try bonus point, and loses by 7 points or less in the match against the Grand Slam team....21 pts