It's not really about the off meta builds though. It's about not giving champions 5 abilities, some with multiple mechanics, which can all be utilized to their full extent with the same build.
Another good example of this is actually AP varus and AD varus. They are both reasonably viable, but where AD varus focuses on a poke centered play style, AP varus has his strengths in medium range burst. Purely based on which stat you decided to build. This means that if you pick varus into a game against a team with powerful engage and/or healing, which you can't reasonably poke down, then you can make the active decision in game to switch your build to play in a why which is more appropriate to the situation. If anything instead of off builds, they should be called "secondary builds", for when the enemy team counters your primary play style.
Actually this might be why lee sin seems to be the bridge between the old philosophy and the new one. He forces players into a primary and secondary build throughout the game. He starts out as an assassin style fighter, then switches into a tank style fighter as he starts to fall behind in scaling. He's got a primary and secondary play style, but he forces players to play in that manner.
I get that they want to make the game "easier to balance", but it's worked to make it harder to balance, when you have to try and balance champions who have this mechanical strength against champions who have numerical strength. Not to mention it's had adverse affects on different archetypes. Increase AD on all items by 10, to help ADCs, then you push bruisers who get maximum value out of AD over a threshold of 100-0 burst.
All they've accomplished is to create a more defined hierarchy of champions and removing player identity in respect to how you prefer to utilize the tools of the champion.
Oh yeah, and as far as AP yi, Tank fizz, Tank Ekko, exc, go. Those were often problems of the off builds getting some kind of unadjustable advantage from core build mechanics. Yi's Q reset, Fizz's E dodge, Ekko's E (all 3 parts) and R. So separate them more, don't remove the off builds because you want to preserve one of the play styles while throwing the other out. This is what happened with Malzahar in the update, except they just combine the play styles into one, in a poorly thought out manner, and now look at the hole they've dug themselves into.
