Yes, Wizard casts Foresight on the EK Fighter. He now is basically unkillable (disadvantage to attack him, shield, misty step, haste, Fighter HPs, Indomitable, Resilience (Wisdom), etc) and in most of our games, did around 70-80 damage per turn, equating to about 400-500 damage consistently (including an action surge here and there) over the course of most combats. He can attack every round, ignores cover, ignores disadvantage in melee, and just tears things to shreds.
Now you could argue the Wizard could cast foresight on himself instead, but when you are playing together as a group, why would you, when putting it on the SS Fighter makes your party do WAY more damage overall.
The Wizard could try and keep up if he shapechanged or true polymorphed into something, but he won't be able to. Shapechange doesn't give him legendary actions, True Polymorph doesn't give him his own spells, and unless he decides to do that permanently he will lose his spell due to concentration. Better to just Foresight the Fighter and be done with it, especially in dungeons and other places that are impracticable for Shapechange/True Polymorph.