Conqueror on ADC [New Rune]

I hope I'm wrong, but I fear I might not be. My fear is the new rune will replace Last Whisper on ADCs that want to avoid that 1300g detour in their late game builds.

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Everyone has been talking about how the new rune (conqueror) is great on top lane bruisers who were previously shoehorned into taking things that didn't quite fit their kits, things like Comet or Spellbook or Grasp. I haven't heard people considering how this rune might be used on ADCs (Marksmen) too much.

You prep it on the wave, and when it triggers you get a free longsword at level 1 in raw AD. A longsword and a half at level 5. Press the Attack is a good amount more damage, but it should be more raw damage, because you need three autos on the same target to get anything out of it, and that's the big part of what Press the Attack does: raw damage. The big part of Conqueror is yet to come:

If you have 150 AD and attack a target with 100 base armor and 200 bonus armor (so 300 total armor) and you will deal 37.5 damage. If you have the same situation with Last Whisper's passive, you will deal 45.5 damage. (8 more damage, 21% increase) If you have the same situation without Last Whisper's passive, but with Conqueror's damage conversion active, you will deal 66.4 damage. (28.9 more damage, 77% increase)

Last Whisper will get better as they stack more bonus armor, but Conqueror looks even better against targets with less bonus armor. 200 bonus armor, 300 total armor is already a decent amount of armor (assuming the tank isn't allowed to heavy stack because of an issue in your team's draft).

I'm afraid that this means ADCs will be able to take this rune and completely skip Last Whisper. Yes, it has down time while Last Whisper obviously does not, but with the way the rune currently works, the down time can be circumvented. Were you clearing a minion wave? Were you hitting dragon/rift/baron? Were you hitting a tower? Were you hitting a rift scuttler? Did you land some poke? Did you attack a ward? Also note that "in combat for 4 seconds" is almost identical to "hitting two instances of damage." A single damage instance you "in combat" for 3 seconds, so if you land another in that 3 second window, you have been "in combat for 4 seconds." With that in mind, answering yes to any of the above questions, which are pretty typical situations, will mean you have the rune ready to go for the next time you hit a champion. The biggest situation I can think of where Last Whisper is much better is when you're 100% out of combat, Conqueror is not ready to go at all, and you must start hitting the tank who stacked armor. There will be a good 3-4 autos worth of time where you could have had armor pen if you had a Last Whisper, but you don't because you took this rune instead. While that situation is not uncommon per se, I think there may be enough ways around it that it can be dealt with/put up with.

Another thing to note is that Last Whisper is only physical damage, obviously. Conqueror is not. That Kai'Sa who just got buffed? When she procs her passive on you with her Q onslaught, it's almost guaranteed 20% of that burst is getting converted to true damage. That Tristana that just jumped you? Again, if she plays her cards right, almost guaranteed that E+Ult is detonating as 20% true damage. That Kog'Maw that landed a single ult, and then pressed his W? Yeah 20% of his hard-to-itemize-against damage is now true damage.

I've heard the argument that "True damage doesn't crit and doesn't proc life steal, so characters that build these will be disincentivized from taking this rune," but this isn't entirely the case. The lifesteal is reduced, sure, but the damage becomes true after the crit. The true damage crits.

It's better on magic damage ADCs like Varus, Kog'maw, Kai'sa. By skipping Last Whisper, you don't have to worry about a slow period where you had to spend 1300g on a Last Whisper. Its effect is certainly less than but still comparable to Press the Attack, an early-mid game 3 hit burst rune, which does not do anything like replace a core item that slows your damage build down.

The one thing I can see that I'm really unsure about is: taking this means it is your keystone (duh). More specifically, this means no Fleet Footwork. The champs that would want to skip Last Whisper are the late gamers who want to scale but had to spend 1300g on that one item that lets them hit tanks. Those also happen to be the same champs that, right now, take Fleet Footwork for the sustain in lane. If this keystone were released with the pre-nerf Fleet, I think there would have been some good competition. But as Fleet is now rightfully toned down, I fear this keystone will be the go-to for scaling ADCs simply because it will let them accelerate their build far more than free boots ever did, by replacing an entire item.

I would love to be told I forgot something, or I'm looking at some math wrong, but I'm not seeing it.

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Also: I timed the prep time, it takes ~5.37 seconds to charge up, not 4. This means in terms of pure uptime it has ~36% uptime if you're just constantly hitting a dummy, instead of ~43%. But that probably deserves its own bug report.

Also also: The true damage conversion wears off before the bonus AD does. Because you know. Why not? It's not like these are part of the same buff or anything.