The lostness must involve eventual, everlasting, painful lostness. But this is not what is being presented in Matthew 16. Such a happy recognition was not then given to the scribes and Pharisees or to the majority of that nation. This is severe enough. But it is not a matter of suffering loss for all eternity in hell.
Soul speaks of the senses and here especially of pleasant sensations.
We say it is unfair for someone to suffer for something they were not given the grace of avoiding. But being lost in sin involves not only present misery but future suffering as well, as God deems best. The problem here is not the existence of future judgment and loss, but in viewing these future losses entirely as ends in themselves and as penalties entirely divorced from divine responsibility and purpose in wisdom. The forfeiting of the joys of the kingdom is a sad thing in itself, but it must be viewed in light of God's operations. That sin has future repercussions needs to be made clear.