First of all, I think you are grossly overestimating the importance of what bayonet is used to the quality of the fighting men we field.
Second; the versatility of a blade as opposed to a purely stabbing instrument vastly increases the weapon's versatility as both a tool and a combat weapon.
I don't feel like going into all the reasons for the switch, and anyway, this is all VERY off-topic, and any other posts after this one not relating to the topic will be immediately deleted.