Is it possible to simply buff or nerf a champion instead of reworking them nowadays?

This. Buffs and nerfs won't solve inherent problems in a champion's kit, _especially if those inherent problems are what makes buffs and nerfs a bad situation for said champion. _

they changed zac because he was inherrently boring to play and maokai realistically can't be nerfed to a balanced spot without making him unplayable in solo que. Which even when maokai was strong I rarely saw him except when he was completely busted. I don't know anything about sejuani but her rework seems succesful. Reksai is kinda the same as maokai where he has super strong ganks and if he's strong he just one shots everyone on his ganks and he's op unkillable tank later or he can't kill people in his ganks and does nothing so they tried reworking him.

Warwick seems to be a success as well and once they nerf zac he might become a success as well but we'll wait and see on that. So there are some failures but quite a few of the reworks are a success and the failed reworks are the ones that if they didnt rework them they would've had to olaf them or make them into a new urgot yorick level of bad. I mean the original Ryze had no skillshots and his spells all gave his spells gave CD reduction for his other spells how do you balance that? he was litterally just a ball of stats either you have stats that can lane bully everyone top lane and become a God or you were to weak to survive all-ins and would just be useless.

That sounds like the True Master Of Getting A Rework (TMOGAR): Ryze.