If it's too hard, people will trade it in.
If it's too easy, people will complete it and trade it in.
I can't imagine it being too hard for single player. I can imagine the balance isn't fun for multi-player. Part of that balance is game time. If the multi-player balance isn't good, people will trade it in.
Either way, millions will sell first week.... It might make a difference how many millions of actual sales, but it is going to be millions no matter what...