"X category's job is to do THIS, X category should NEVER do THAT"

That's never how it works. There's a general thematic for each category, and then there are obvious and intended differences. Do you want all your melee champions to be Malphite and all your ranged champs to be Varus?

And same for builds. There are main builds because there are most likely desirable strategies for each champion, but there can at times be other reasons to pick a champion than the most frequent, or other needs that develop within the course of the game.

This is supposed to exist, and it has never been a problem. The problem appears only when a champion builds for a role that isn't their intended one - but the gains in other sectors far outmatch the losses in the intended role. The problem appears when a squishie building bruisish still stays among the highest DPSes/bursts the roster has to offer, or when a tank building semioffensive items gains enough defenses from them to have his/her defensive scalings be effective.