If you would NEED to have Rogue with you, since noone else is able to lockpick, stealth and disarm traps so effectively ... if you would NEED to have Cleric with you, since noone else is able to heal, buff you and debuff enemies so effectively ... if you would NEED to have Wizard with you, since noone else is able to cast spells from scrolls so effectively ... if you would NEED to have Fighter with you, since noone else can jump so far, handle heavy objects etc ...
Or you know you could design RPG with actual choice and consequnces. With various valid play styles, different exploration options for different team compositions. I don't need to be able to speak to the Dead or animals if I didn't pick certain classes. I don't needs all my party members to jump like rabbits all over the place. If I pick all melee classes I don't expect them to also double as archmages.
With how much "depth" Larian put in the game, each playthrough feels very samey - and that's because interaction with the world doesn't change.
You also forget that there will be multiclassing - if you want your fighter to also cast weaker spells then dedicated wizard - you will be able to do that.
