Why Ap Items need a cap

So I was recently playing a game of ascension and it was against Diana, ahri, and akali. at about 15 minutes into the game, I was one hit by the enemy Akali, and was just amazed and like wtf what just happened? Then I began thinking, what do Diana, Ahri, Akali, Katarina, etc. have income? Katrina is on a perma ban list atm btw. And the rest deserve to be. Then it hit me, they all are what Yasuo use to be. I don't know how many remember the days Yasuo was built with AP items, but those who do can tell you he rolled through the enemy team as if they were butter.

Now we have basically the same thing happening again. I mean Diana, Akali, Katarina, and Ahri all have huge amounts of mobility, like Yasuo but theirs isn't minion or champion reliant. Each one of them has the move sets of AD champions, but are built AP, like Yasuo was once. They are all speed champions, meaning their attacks are instant. They can build enough ap to do real damage instantly and roll through the teams, and build items such as life steal to counter any chance of being outplayed.

Take into account Yasuo. Yes he is super mobile, but with no minions in a 1v1 he falls short, especially against these four whose dashes, shield, teleports, etc. are not reliant on there being someone to run too, a build up of a energy bar, nor long cool downs. Plus the fact that they have much higher defenses then yasuo, better shield, stronger dashes, etc. doesn't help.

Now imagine this for a minute. Unnerf yasuo. Bring back his his health which was removed a while back. Boost his mr and armor, like he once had. Make him ap. Add a longer shield. What do you get? You get a super broken champion who will roll 5v1 through any team. Now take that into account and realize that's what Katarina, Ahri, Diana. and Akali are. Buffed Yasuo type champions, who can one hit you, are fairly good, or at least better than Yasuo's early game and late game.

So why when Yasuo was so broken people complained, but no one cries about the rest of them? I mean At this point with magic items having received huge amounts of cdr, having up 120 ap, having a lot of effects on their items (slows, stuns, etc.), what makes it okay for these type of champions to still be allowed to run loose. Katrina has near 100% ban rate on every game mode i've played so far.

There needs to be a power cap on ability items, just as there is on ad items. Max of 85 ability power. I mean how is it fair the highest ad item is 70/75 at most and the highest ap item is 120 and you have many ap champions, like veigar, being so op that he can literally one hit someone if he's played by a good player.

Imagine making Yasuo or other champions AP compatible again, now tell me why is okay for others to still have this ability?