A tank guarantees nothing. Look at Edmonton. The only reason Pittsburgh tanking worked is because of Crosby and Malkin. You think they were happy with J. Staal and, especially, Fleury? Chicago netted 2 great players in Toews and Kane, but their roster building has been a very important component of their success.
With a tanking philosophy, you're talking about a 3-5 year commitment. Most fans dont have enough patience for 3-5 bad games. Thats not even getting into the issue of having to make sure that great, franchise changing players will be available with a top 5 pick.