We don't use energy like rogues though, have you played a rogue?
Rogues are extremely dependent on their energy rotation, they do not have an "infinite pool that is never expended".
Managing mana frankly was never interesting for Frost or Fire and I don't think I really know anyone who misses it. People only miss Evocation for the heal, not because they were forced to cast it or go OOM (let's note that if you make evocation necessary to avoid running out of mana, people wouldn't be able to use it to heal anyway).
It was also never "iconic" for mages to use mana because so did literally every other healer and spellcaster.
Removing old mechanics because they don't actually do anything useful or interesting is good design.
We've already been over why a raid-wide temporal shield could never happen so I'm not going to bother bringing that up again.