Mana changes are probably intended to shift mid's primary goal away from lane priority
Most of what mid does that's useful in this meta is secure priority for the jungler to invade and secure priority to roam bot and snowball bot. This basically means mid is currently very team-reliant in how it can impact the map, since most meta mid matchups aren't about the 1v1, but who shoves in who faster. Rito probably is hoping that reducing the amount of available mana will butcher this playstyle because it sucks for solo queue and severely limits the mid pool in pro.
