So, I should go with the item that will do me no good at all if I am dealing with high-range poke champs which are very common in mid these days and are roided up in this mode with items that increase their AoE damage, kiting & chasing ability, or evasion abilities because 10% lifesteal will make me invulnerable.

And I should do this because my all in will be more effective with an item that costs more than Pickaxe and longsword, which combined grants 15 more AD and are cheaper and builds into an item that only comes 1 kill or 3-4 minion waves later than Flesheater which will grant at least 15-18 more AD on purchase, speed up my farming so I can roam and gank lanes more easily, add an animation cancelling move with a decent reach that's worth additional 60%-100% of my AD to my burst combo that is on low cooldown and can also help farming, and makes my AAs burst for 20-60% of my AD to any target close to my target when I am bursting them down.

You are right, the item that starts with less AD than a Pickaxe and punishes me for doing the primary strategic point of an assassin mid-laner rather than helping it is entirely worth it for the 10% lifesteal that doesn't do crap for you in team fights and does not compensate you well enough if you are against a high poke lane in this mode that roids up AP poke and AoE.

So... okay. Maybe I get Tiamat after? Let's say we'll have it stacked to 40 AD by Tiamat completion to be nice. For 40 AD and 10 lifesteal I have delayed getting the next item that grants 30 AD, 10% CDR, 15% Critical Strike chance, 20 Armor Pen, and a 45s cooldown ability that gives me 40% attack speed and 20% move speed (which is good for getting in and getting out, thus is good for roaming and spilt pushing). Oh and Tiamat (a core item on many melee ADs) impedes stacking because it kills the wave faster.

Oh I can get Youmouu's later right? Which delay's Last Whisper. Let's just say we've managed to stack up the AD for the item by this point just to be nice again. so we are only losing 35% pen that we would otherwise have at around this spot for that 10% lifesteal.

I could keep this going to illustrate how each delay impedes the effectiveness of your roaming, teamfighting, and farming as the passives and activates you are delaying are far better than a mediocre level of additional level of flat AD but let's just cut to the end.

For an item that is less effective than every other item an AD assassin builds unless you spend most of the game farming rather than pushing up your lane and roaming, teamfighting, and securing objectives you lose one of the following last items that are common last items on AD assassins (skipping things like IE that would likely be an earlier than last item): Mercurial Scimitar, Guardian Angel, Banshee's Veil, Black Cleaver.

You lose those kinds of utility benefits for an item that becomes useless if are losing lane or in a bad match-up, punishes you for doing your job as an assassin mid-laner, and does nothing but give you a bit of extra AD and 10% more lifesteal.

So yeah. I'll just snowball by pushing objectives and helping my team snowball thank you.