I think the Marble tourney (one of the harder fighting tourneys) in the summer in the middle of a major event is a bad data point, but I generally agree. The 10th chest is out of reach for most players; my last FS, which was mostly casual players in Dwarves-Orcs, everyone had to save up for a month just to make a push, and we still didn't get it. So yes, the 11th chest would be seen by very, very few players. So I agree, I don't think the 11th chest is a good use of developer time (not only to program, but also to balance, and to invest prizes).
I'm just saying as a suggestion, its not insane, because the nature of the game (via AWs mainly) is that the tournament gets easier. So if the tournament is supposed to be a challenge, at some point (maybe now it would favor the top 1% of FSes too much) it would make sense to add more levels. Maybe 6 months, maybe a year?