Let me tell u a little story about diana

She isnt an assasin. She belong to diver class -according to RIot-that its design to dive into the enemy and maybe kill someone with her burst but havent any reliable way to get out of any sticky situation.

pre rework akali and diana are similar yes, but not the same. Its like saying omg ashe has the gameplay as miss fortune!!!! they are both adcs and they can attack!!!! and their ults are important!!!

but in reality they were quite different. I did use to main both of them at some point. Diana's early game (pre 6) is weak-ish but you got wave clear. Pre rework akali's early game wasn't weak, it was ok. She wasn't a lane bully but she wasn't weak!!!! which is why usually you could get like a kill pre 6. And outside of your Q, you had to be in melee range to clear minions due to no wave clear.

Why the difference? that's because diana has no mobility outside of flash pre 6, while akali had her W blink.

And even post 6 there's was a large difference. Diana, the prime assassin, she can burst targets in 1 second or so. Akali on the other had, she needed more time to actually burst someone down. Then was akali worse? No, because she had no mana and she could chase much better. If Diana missed her combo on you or you disengaged properly, you are pretty much safe and she won't be able to properly chase. With akali, nop, she could chase you with 3 long dashes and 1 short one.

Also, akali can build items that would heal her, while diana had to reply on burst and zhonias. So, the way they approached late game was quite different