I think you missed that I play twitch as a jungler, not as the typical bot AD w/ support babysitter. This changes a few of your assumptions-
-I need smite, so no cleanse/flash combo. Also, I know this is probably a minority view, but I prefer ghost over flash. Early game flash is stronger, when you can flash from mid-lane to your turret*, while late game ghost is stronger because it gives you a lasting speed increase and lets you catch or get away from nearly anyone.
*Slight exaggeration, okay.
-I don't need to stack damage like crazy. My team probably has another ranged AD running bot lane with a support who can fill that role.
That also said, once you reach 180-200 AD, getting more damage through adding more AD isn't nearly as cost effective. Increased attack speed and increased crit percentage makes a larger difference on damage output at this point.
They all scale off of each other, so a simple blanket statement isn't really telling the whole story. Simplistic view, if you have 100% crit and 300% of your normal attack speed, each point of damage is worth 6 times as much as a point of damage at base attack speed and 0% crit. This is ignoring things such as crit damage increase of mysteries or quints, enemy armor, and dozens of other factors. There is also the fact that you can not choose exactly which stat you want to buy at any given moment- you are limited to the constraints of 6 item slots, and limited items such that you can't just build a pure item with one stat only, nearly all items have multiple stats or effects.
It would be interesting to see someone do a thorough analysis of the benefits of each stat over the other at various points in the game, but even that would be of limited usefulness because different champions and different builds would change the results.
30% crit chance from PD + 15% from EC + 25% from IE is a massive damage increase to any AD champ. 2 out of 3 attacks being a crit is huge.
It is, but is it always worth it? I've been super fed and used builds similar to your suggested build, and a malphite ult + sion stun + amumu ult and I am completely shut down during a team-fight and killed without doing anything useful. I've found there is diminishing returns on stacking damage without resists on twitch.
I think it comes down to the strengths of the champion. Stealth, as an ability, is the ultimate initiation tool. You can position yourself behind enemy lines right in place to kill their carry, but if you actually initiate from this position you are in no place to escape should things turn ugly. Best case scenario, you position yourself, your team's real tank initiates from the other side of the enemies, fight ensues, you come out of stealth and attack. Even with this best case scenario, it's not hard for a focused enemy team to simply turn around and kill you first, largely ignoring your team until you are dead. If you have a pure DPS build with no survival, you get CCed, die, and that is the end of the story. If you have a tank DPS build, you survive longer, possibly take someone out, and your team has the time to destroy the enemy team while they focus on the twitch instead of the real hard-carry ranged AD.
If you stay out of range and never go behind enemy lines maybe you can do okay with a pure DPS build, but you might as well not be playing a champion with stealth.
Looking at my recent games history, these are builds I won with-
doran's blade, ninja tabi, doran's blade, zeal, aegis, cloth armor
went 7-0-4 Based on my items, I must have been playing vs an AD heavy team.
doran's blade, ninja tabi, aegis, doran's blade, maw of malmortius, wit's end
7-1-8 Based on items, must been up against a mostly balanced team with a strong burst AP caster.
phantom dancer, merc treads, banshee veil, maw of malmortius, wit's end, bloodthirster
23-12-12 Based on my score and items, very long and bloody game with deaths on both sides, and I didn't seem to care at all about AD so the enemy team must have been heavy AOE/casters.
doran's blade, boots of mobility, wriggle's lantern, zeal, executioner's calling,the black cleaver
10-0-6 One game where I basically ignored defensive items, it included some very strong early game ganks that snowballed into me being able to take on anyone on the enemy team without fear, so no need to slow myself down with resist items.
And losses-
executioner's calling, ninja tabi, phantom dancer, giant's belt, infinity edge, last whisper
6-10-12 Looks like a long game, and I didn't have much a lack of damage. I suspect I was getting CCed and killed, probably needed a banshee veil.
phage, ninja tabi, wit's end, hexdrinker, wriggle's lantern, thornmail
9-9-6 Thornmail wit's end hexdrinker- looks like I was trying to cover both resists and armor but couldn't get enough of both. Or maybe my team just let me down, usually a break even score on a long loss means someone else on the team was feeding bad.
doran's blade, ninja tabi, wit's end, frozen mallet, zeal, wriggle's lantern
4-13-10 I did terrible, not sure why. This was just about my cookie cutter basic build of wit's end + wriggles + frozen mallet, but it's obviously not unbeatable.
trinity force, ninja tabi, hexdrinker, wit's end, wriggles, banshee veil
13-12-14 (barely) positive record on a loss, I feel this must have been a team letting me down or possibly just a 4v5. Looking at my items I think I remember this game as being vs a malphite/amumu/morgana heavy aoe magic team, but even stacking magic resists wasn't enough.
black cleaver, mercury treads, infinity edge, phantom dancer, banshee veil, bloodthirster
13-15-30 A very long loss, had just about every item I could want. IIRC it was a terrible matchup for me as twitch, looking at the enemy team I saw 5 champions that all counter me.
Meh, I don't see much of a pattern on the items, except that I don't build wriggles or EC very often anymore. That, and my lost games last much longer on average than my wins.