The Beastmaster isn't as good an example of a strong late game mage as a Priestess because his defenses aren't nearly as strong, and it's harder for him to keep his investments alive long enough to reach a dominating late game position. Although just a bear and multiple wolves is hardly a late game position anyway. You can reach that state by turn 3-4, although you might have to accept an unhealthy lack of defense in order to pull it off depending on what your opponent is doing.
Turn 1 (19): Ring of Beasts -> Steelclaw Grizzly (1)
Turn 2 (10): Enchanter's Ring -> Timber Wolf (0)
Turn 3 (9): FD Brace Yourself -> Timber Wolf (0)
A dominating late game position might be something like 2 Guardian Angels, Samandriel, Brogan, and a Knight of Westlock. Playing that much total creature strength is completely unnecessary against a solo rush build, but can be absolutely vital in an attrition mirror match. It really depends on the matchup.
3 Knights of Westlock is more like a midgame position than a late game position unless they had a lot of enchantments boosting them or something.
As to whether attrition strategies are viable in a competitive environment, I don't know. I haven't heard of many in tournaments, but the tournament sample size is so small that it's hard to draw firm conclusions about the relative strengths of different strategies from the data I've seen anyway.
I'm also not sure how long most people that play at tournaments would take to win with an attrition strategy, which could impact tournament viability. It shouldn't hurt the Attrition Priestess too much due to her tools for gaining a life advantage, but I can see it hurting other attrition mages.