I can speak for the mage side.
The intention with these changes is to make builds more differentiated from each other.
We also didn't want to make the system more difficult to balance (ie. have to ceiling one subclass' access to haste based on another subclass' access to haste).
From the design side, we want to use Haste as both a "progression" axis (makes your champ feel like it's getting stronger relative to the game state) and "choice". The AP Mythics all having Haste are intended to make your champion feel more fluid (you're not meant to choose between haste and no haste on Mythics, but instead a little haste [Ludens] and a lot of haste [Liandrys]).
As much as possible, we wanted to avoid the "one item that fills everything" in the second slot. Cosmic Drive is intended to be the "choice" Haste option, fighting against some of the more burst oriented options like Horizon Focus, Lich Bane, scaling options like Archang...
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