AP versus AD

Am I the only one who feels that the difference between these has become so wishy-washy there's barely a point in the two?

My understanding was in the beginning AP should burst, doing damage quickly and AD was more damage over time. But basically any quick damage assassin, Zed, Yasuo, Basically any top laner, does just as much burst, PLUS can farm with auto easily and is usually also super tanky.

Late game any AP champion who isn't complketely fed and isn't named Teemo (maybe Eve now? Not sure) is uselss.

Plus there's Armor Pen AND Lethality. And a grand total of 3 (3.5 if you count wits end) magic pen.

There's almost no reason at all to ever play an AP mid these days. Zed, Yasuo, Talon will just make Leblance, Ahri, Brand eat their own asshole.