Mana-gated laners who have lost lane control must choose to either tank the wave and lose health, clear the wave and lose mana, or last-hit the wave and lose turret health. Winning matchups, whether due to a counterpick or earlier lead, can wear their opponents down by forcing them to repeatedly make this choice.
However, Irelia does not spend enough mana clearing the wave for this choice to matter. Bladesurge and her other skills do not cost enough mana. Therefore, even when she loses wave control, Irelia can choose to clear the wave without being worn down.
-Bladesurge: Cost increased to 20/25/30/35/40 Mana.
-Flawless Duet: Cost increased to 80 Mana at all ranks.
Increasing the cost of Bladesurge with its rank would require Irelia to spend mana to use it for waveclear, forcing her to make the choice other mana-champions must make, while also punishing her more for using it without a mark or unit-kill, and require her also to have mana for an all-in. Increasing the cost of Flawless Duet would further increase that cost, as well as the cost of fishing for an all-in.
Overall, increasing her mana consumption, especially early, would endanger her to being pushed in and then bullied by midlane mages if she does not retain control. Since she skirmishes and scales well, and has good all-ins throughout the game, she would still have many paths to victory. Indeed, nerfing her midlane performance would permit buffing her toplane, where she began.