Ofcourse it wasn't designed for Ekko! Ekko didn't even exist when Triforce was put into the game!
Okay now that that's out of the way, an item shouldn't be deleted just because you feel it's too strong on certain champions. Triforce is a core item on Irelia and without it she would be hard pressed to find a suitable replacement. Infinity edge was never desingned for Shyvana, yet it works for her, but doesn't make her over powerful.
Champions are all going to have natural strengths and weaknesses when you get certain items on them. If it proves to be a real problem Riot will do something to fix it.