If the only argument is that Diana is not toxic if Ori is not, then I can somewhat hop on this idea.
I won't argue that Diana does more or less damage, since she is a burst assassin while Ori is a much more rounded damager (low cooldowns with moderate damage).
I simply believe Diana needs adjustments to her kit.
definitely lacks a strong sense of counter play, in that she does almost everything (Shields, Hard CC, ranged poke, strong wave clear, decent pick potential), and with her movespeed buff, decent shielding (240 + .4 AP), and ranged attacks, she definitely seems like a top-tier pick for the mid lane. I am not an avid Orianna player, but from an outside perspective** I can't really name a glaring weakness** that she has.
, as has been mentioned in a lot of other posts, seems to lack a solid healthy identity. A lot of her damage is almost guaranteed if she is able to ult to you, her dash is much faster than an Orianna ball (I don't have exact numbers on Diana's ult projectile speed but it seems faster), and her burst scales better with AP. Meanwhile, she is melee, she does not gain defensive stats (WHICH ORIANNA DOES!?), and her combo is interruptible (Thresh could flay her mid Ult and theoretically nullify almost all of her damage). Because Diana's initiation is interruptible and requires her to go all in, it means she is not a reliable initiator for teamfights or picks, and since she has no reliable escapes (she requires a target in her escape direction) unlike Lee Sin who also has an escape that can be used on wards, she lacks a strong tank presence because once her ult is nullified she can almost be ignored or easily kited. Most currently viable initiators have either a relatively safe ranged pick potential (Elise, Morg, Orianna, Syndra, Lee Sin, Ryze, Mundo?, Rengar?) , or unstoppable/highly reliable initiation with decent beef (Vi, Maokai, Alistar, Evelynn) .
As a tank
is unreliable and lacks the inherent beef (No passive defensive stats other than her decent base stats) given to most tank-like champions, in addition, her shield at max rank costs 100 mana (Which is a lot because she has borderline Sion levels of mana); and without putting herself in danger, only blocks 100 + .3 AP (200 + .6 when able to proc the second half). It's hard to compare her shield to others since she is the only melee tank/assassin where the shield itself does damage, unlike Nautilus and Sion where the shield must be maintained to do damage, Diana's damage comes with the act of activating the shield. But generally, this move is used as an attack spell rather than a defense spell, because when comparable defensive shields are nearly free (Rumble, Shen, Lee Sin, Riven) or scale with health in some form (Vi, Malphite, Nautilus), hers seems to be a mismatch of ideas, causing a mediocre spell with only one viable use case: to attempt to kill someone with it. With such a high cost and low shielding without going in, Diana is forced to engage to gain a reasonable shield or simply forgo wasting the large amounts of mana and take harass. As a melee champion I feel this severely handicaps her abilities in lane and limits her decision making. I understand this may be the type of character Riot was originally designing Diana as (Hyper all in), but in terms of balance I feel it doesn't work as well as it could.
Maybe Diana's shield could start with the max shield, and if the bubbles pop give her an Armor/MR bonus? Possibly lowering the base damage on it and tweaking the scaling while lowering the mana cost to make it more usable in lane.
As an Assassin, Diana's damage is uncounterable without a displacement, meaning if she is ahead, you will die, similar to fizz, akali, or talon, but fizz, akali and talon have an escape for trades and losing situations in lane. I really don't have much else to say on Assassins because to be honest, melee assassins are unreliable in nature.
I'm no pro on either of these champs, I'm just sorta rambling for fun.