I guess it depends how you see the role of the coach.The job of the players is to get that premiership!
Everyone around them has a role to play, and head coach is definitely a key role, but all their roles are to help the players win ... and the players aren't always up to it!
(Greatest coach of all time has his team sitting ... second last!)
For me, he (or she but not in NRL) is most like the conductor in an orchestra, the final and most important element. An orchestra, like a sports team, can be brimming with the word's greatest talent, brilliant individuals. But without the conductor (here we go with the dreaded puns) being on song, the end result is decidedly off key. Same with any sports team.
You could argue that the difference is that the coach isn't on the field, but he is in the box and should be able to respond to what he sees in front of him. The most glaring recent example of that from Seibold was clearly being outcoached by Adam O'Brien who made moves, mid game, in response to our weaknesses, without Seibold making a single counter move.