Actually, let's use that Hadoken example a bit more:
Let's say Player 1 shoots a fireball, right? Advantage is a ranged attack, but the downside is that they're left doing the follow-through animation of the attack after it leaves for x frames. Doing it closer gives less reaction time, but doing it farther away makes it easier to dodge.
Player 2 is now reacting to that fireball. If he were to simply block, blocking in and of itself is instantaneous. The downside is that you take chip damage from special moves + you're left in a state of block stun where the guy on offense can continue to pressure you, but you can rest easy that you either fully block or mostly block incoming attacks (unless thrown, but that over-complicates the example).
Now let's translate this to League. If I were to use a shield while someone's throwing out...let's say a Ziggs ult, the shield comes out immediately, and the advantage is that I get to block the damage of the attack as much as the shield allows. We can relate LoL shields to fighting game blocks.
Now let's say Player 2 jumped, then tried to attack back. For this, Player 2 could clear the attack but miss his own attack, clear the attack and land his counter-attack, or just fuck it all up. With this said, the properties for jumps in SF start up instantly, as well. You're immune to throws as soon as you hit up, even if your character isn't off the floor yet.
Now, let's translate this back to our main point. Fiora's W works a lot like that jumping response (I mean, it ACTUALLY works like a counter, since it's a counter, but Idk how much you know about fighting games). The defensive portion of this reaction, the ability soaking wind-up, needs to start up instantly because it's mainly used for a reaction to an incoming attack. The OFFENSIVE part of this move, the stab, behaves like the kick after the jump. It COULD miss. It COULD hit. It's not instant. It's not protected. The offensive part of the move is telegraphed and dodge-able, but less so for the person she's countering. It's her reward for using the move properly, similar to how it's foolish to just randomly jump in SF unless you want to eat Shoryukens to the crotch on repeat, but doing so properly allows you to dodge and land hits you wouldn't be able to all while avoiding chip damage outright.
The point I'm getting at here is that a defensive move needs to be snappy because what's the point of a defensive move if you have to predict farther ahead of time than the person that's trying to decide how to attack you? Offensive moves, on the other hand, have wide variety in terms of delay based on the payout and purpose of the move. Fiora's W has both these concepts playing into it at the same time in ways that make logical sense. There's no need to tack on extra conditions to it.