If riot nerfs MP5, New champs will continue to overshadow old champs
Mana is a gate to champion power. Riot decides that gold tradeoff for more power is effective. While in its current state mana bound champs are relatively healthy, problems arise when comparing old v new champions. New champions rarely have appropriate mana cost/mana pool/MP5 balance relative to older champs. the simple way to solve this is to change the older champs to better suite the new way riot wants mana champions to be (get an item and spam spam spam). Even though the diversity is important, the perks of older champs aren't something that make them invincible to nerfs. Although knowing Riot they might just nerf MP5 items and call it even. further barring out old and mana hungry champs, furthering their need to invest in more mana items to keep up. further more putting manaless champs at an even greater advantage early-mid game.
Riot has put in effort by lowering mana costs throughout the season. specifically in ultimates. Yet they may over look this idea when they announce "major testing changes" that could potentially destroy certain champs from being viable during the major changes.