What is the point of a buff...if we're simply going to negate it?

For example, Evelynn, as she is a hit topic. Her passive was changed from 1% of her mana to 2% of missing mana. Its a buff for when she is below 50% of her mana pool, but a nerf for anything above it. It was meant to prevent her from bottoming out too easily and ruining her early game. People complained but the change went through and her mana remained an issue, but we'll say it wa sa buff.

They promptly tack on a mana cost to W because Runeglaive was helping her to also maintain mana. So...why not revert the change that was implemented? If the 2% maximum mana change was meant to prevent bottoming out, then return it to 1% of maximum mana. It would be a nerf to her mana when she is low anyway and counter runeglaive's regen.

This is the same for other champs such as Yasuo, Vayne, Lee Sin, etc etc. Why negate the buff, and in some cases add additional nerfs, instead of reverting the previous change as a way of dialing back the changes? Why experiment and introduce additional factors that only complicate the issue?